BABBA issue # 14 - April 1994, page 12




Are You Looking SCSI Lately?

Part 3: Installation of SCSI Systems


(By Fred Townsend)

Originally, disk controllers were huge refrigerator-sized boxes that controlled the washing machine-sized hard disks. By the time IBM designed the XT, disk controllers had shrunk to single printed circuit boards. As the evolutionary process continued, some controller functions migrated to the ESDI drive. Today, IDE/AT and SCSI drives contain all of the disk controller functions.

The IDE/AT and SCSI drives still need an interface to their host computer. Since these interfaces no longer contain disk controllers they are now termed host interfaces or host adapters. This article deals with the installation of the host adapter and the SCSI devices connected to it.

Home from the Store
OK, you made the plunge and purchased your first SCSI components. Now you are ready to assemble them into a SCSI system. What do you do next?

SCSI is a complex system, but with a little bit of planning, installation can be a painless process. Don't worry about already existing hard drives. It is not necessary to remove MFM, RLL, ESDI, or IDE/AT drives. SCSI host adapters and devices are designed to coexist with almost any other type of peripheral device including other SCSI systems.

Usually, placing the card in the box is not the problem. For most do-it yourselfers, its setting addresses and interrupts that muddy the installation process. History repeats itself each time an expansion slot is filled. Good news! Most SCSI host adapters simplify this installation process and may even alleviate some types of interrupt conflict.

Most SCSI host adapters require BIOS entry, interrupt, and I/O addresses as well as a DMA channel. If they include a floppy disk controller, they require an additional interrupt and I/O address. Once these requirements are met, adding additional SCSI devices is a breeze because the interrupts and address are already set.

Unlike a sound card or proprietary tape interface, which may share addresses, SCSI host adapters tend to default to the unused addresses and interrupts. Their addresses, usually, will not conflict with existing devices and so moving jumpers on the host adapter is not required. Many newer host adapters use soft configuration and never need jumper configuration for anything.

SCSI BIOS Address
SCSI BIOS is memory mapped to an entry point between C8000 and F0000. (Note: The trailing zero is frequently dropped from BIOS addresses so C8000 may be expressed as C800.) Most Adaptec and BusLogic SCSI host controllers are shipped with an address of DC000. This address will not interfere with other host controllers such as RLL or ESDI that typically use C8000 or D0000. Most MFM or IDE/AT host adapters do not require an entry point.

SCSI Interrupts
Typically, host adapters provide interrupt numbers between 9 and 15 (except 13) with a default set to 11. Non-SCSI adapters typically default to 14 or 15. Again, this does not normally interfere with existing hard disk host adapters. (Using interrupts 9 or 10 is not recommended. Many machines use these addresses for interrupt cascades.)

DMA Channels

Many peripheral devices, including some SCSI host adapters, do not use DMA (Direct Memory Access) channels. However, DMA access is one of the better methods for improving peripheral performance, so having selectable DMA access is desirable. Most Adaptec and BusLogic SCSI host controllers are shipped with their DMA channel set to 5. (Local bus or PCI bus host adapters use DMA but do not use a DMA channel for access.)

I/O Addresses
Almost every peripheral device is memory mapped to an I/O (Input/Output) address. Host adapters are no exception. If they contain a floppy disk controller, they require two I/O addresses. The floppy disk controller I/O address may not be selectable on many host adapters.

Embedded Floppy Disk Controllers
Many hard disk host adapters contain embedded floppy disk controllers. Except for sharing a common bus connector, the controllers are independent of the hard disk controller and may be used or disabled without effecting existing hard disk or SCSI devices. If there is a previously existing floppy controller and the new host adapter contains a floppy controller as well, one controller will need to be disabled. (While it is theoretically possible to use two floppy controllers on two host adapters, it is not practical.

Attempts to use two controllers may significantly affect system performance.) When two different floppy controllers exist, the choice of which one to use requires some analysis. If the original host adapter is to be used temporarily for file migration, then the floppy controller on the new host adapter should be used. Also, if the original host adapter is a MFM type, the floppy controller on the SCSI host adapter will probably give better performance. If the original floppy controller is embedded in an IDE/AT host adapter and the host adapter will continue to be used after installing the SCSI host, then it is probably simpler to disable the floppy controller on the SCSI host adapter.

Most floppy controllers are enabled or disabled with jumpers. Some of the newer host adapters use a soft setup where the jumpers are replaced with commands from the keyboard. Such soft configurable cards require special consideration and planning. For instance, if the installation process required booting from a floppy because the hard disk was not installed yet, and if the floppy was not accessible because of address conflict between two controllers or the soft select had previously been turned off, then it will not be possible to boot. This situation can usually be solved by temporarily removing one controller or by restoring the soft select to a default configuration using a master reset procedure. Not all host adapters have master resets or ability to disable the floppy controller. It may be necessary to disable those special cases by setting the controller to an alternate address.

Installation
Start hardware installation by making a block diagram of all SCSI components. Include all hard and floppy disks, even if they use non-SCSI host adapters. Add all network interface cards, tape interfaces, sound cards, etc. that use I/O or BIOS addresses, interrupts, or DMA channels. Also include a wish list of any future SCSI devices.

A little preparation now will save hours on installation and future expansion. Label each host device with its appropriate I/O or BIOS address, interrupt number, or DMA channel.

Now, assign SCSI ID addresses to each SCSI device. SCSI IDs range from 0 (zero) to 7 (seven). Determine if booting from a non-SCSI disk or SCSI disk. When booting from SCSI, most systems will assume SCSI address zero contains the boot tracks, so assign this ID to a bootable device. If dissimilar disks are used, a slight increase in performance will be obtained if the fastest disk is used for booting.

Normally ID address 7 is reserved for the host adapter. Assign IDs 5 and 6 to tape units, scanners, or CD-ROMs. Save the remaining lower addresses for future hard drives.

Caution: MSDOS and other operating systems designate both logical and physical disks in a similar manner. Both physical and logical hard disks, including RAM disks and CD-ROMs, are designated with letters starting at "C:". The re-use of the same letters can prove to be terribly confusing.

To minimize confusion, do not assign drive letters to drives or CD-ROMS. Rather, assign the manufacturers designation or model number to each block within the diagram. When formatting drives, assign the same designation to the drive volume label. When using a pair of like drives, it may be convenient to combine alias names such as "BOOT" or "DRIVE_2" or "TAPE" to produce volume labels such as "LX540_BOOT" or "LX540_DRV_2".

SCSI BIOS and installation software normally work with operating systems as old as MSDOS 3.0. However, these older operating systems limit partitions to 32 megabytes. Partitions of this size may prove to be a nuisance. Consider, carefully, whether partitions are necessary for drives larger than 32 megabytes. If so, then an OS such as MSDOS 5.0 or newer should be used. When switching operating systems be sure the new OS is compatible with any compression programs such as STACKER, memory managers such as QEMM, and tape backup systems that are in use.

Next, consider how the SCSI blocks will be connected. Most SCSI host adapters accommodate both internal and external devices. It may be convenient to keep present drives internal and use external locations for the additional drives.

Cables supplied with host adapters usually support a maximum of three devices. It is probable that expanded or future systems will require cable replacement or augmentation. Appropriate cables may be used with all seven devices, or the devices may be split between internal and external cables. Cables for more than three devices are difficult to find and may require custom fabrication.

Copy the information contained within each block of the block diagram to paper stickers. The information on the tags will aid installation and will be invaluable to a service technician or yourself when upgrading your system. Make up tags for the projected future additions too. Place the tags on the back of the devices, or in the case of the future devices, somewhere inside the cabinet. If possible, avoid placing the tags under the wide signal connector since this area will be hard to read when installed.

Using the appropriate documentation, configure each SCSI device. Hopefully, you obtained and retained diagrams with each SCSI device showing its addressing scheme.

The SCSI specifications deal at great length with logical addressing of SCSI devices. Unfortunately, no effort was made at physically standardizing the location of SCSI ID jumpers. If you lack this documentation it may be available on the manufacturer's BBS or your favorite local BBS. Otherwise it may require some exploration and experimentation to determine the addressing scheme used by your SCSI device. (Host adapters normally have their SCSI IDs permanently set to 7.)

Lost the Docs?
(The next paragraphs deal with determination of addresses for SCSI devices without manufacturers documentation. Skip to Software Installation if all SCSI addresses have been successfully set using documentation.)

Some SCSI device manufactures make the ID jumpers very obvious. Other manufacturers will obscure the jumpers by placing them among like jumpers for other SCSI options. Fortunately, a SCSI system may be used to read its own jumpers. The trick is to locate the possible jumpers and then remove and replace them, one at a time, while noting their effect.

Start the procedure by carefully mapping the location of the possible address jumpers in case you need to return the device to its original state. On 5" drives the jumpers are usually close to the large wide signal connector. Sometimes, they will even look like they are part of the signal connector. Frequently the address jumpers will be separated into 3, 4, or 5 pin pairs near the power supply connector. 3.5-inch drives usually do not have room on the rear for the address jumpers so they will be found on the bottom of the drive.

Make sure the host adapter is correctly installed. Prepare the unknown device by positioning it near the host. Connect the power to the device but do not connect the signal cable. If the computer has a slow/fast switch, set it to slow. Attempt to boot the host computer while carefully watching the screen. Somewhere in the boot cycle the SCSI BIOS should announce its presence. Shortly thereafter, the host adapter should start interrogation by stating, "Searching for SCSI target 0, LUN 0". Since no SCSI devices are connected, its search will fail and eventually it will state something to the effect of nothing found.

Turn off everything and connect the SCSI device to the host adapter using the connectors closest to the ends of the cable. After connection, again attempt to boot. This time, after the Searching for SCSI target 0, LUN 0 message appears, the host will discover the SCSI device and announce its model and target number (SCSI address).

If the SCSI address is not correct, power down the system and change one of the suspected address jumpers by adding or removing. Repeat the boot-up determination procedure until the correct address combination is determined. Warning: Do not attempt to move jumpers with the power applied. (Jumper settings are only read at power-up, so moving them with the power on will have no effect.)

Remember the jumpers are binary encoded, which means they are assigned the values of 1, 2, and 4. No jumpers installed is equal to an address of zero. An address of 3 is set by using a combination of jumpers 1 and 2. Address 7 is reserved for the host adapter, so jumpers 1, 2, and 4 should never be present at the same time. Also, jumpers may be used for functions other than SCSI addresses, so if after changing a jumper there is no effect or the host adapter fails to recognize a device, that jumper is probably not an address jumper. Drives are frequently shipped from the factory set to address 6 (i.e. jumpers 2 and 4 installed).

CMOS Setup
The IBM AT's CMOS setup frequently has been a source of confusion and bewilderment. The issue of physical drives, logical drives, and controllers is a major source of this problem. The inconsistency of CMOS precludes a setup procedure in this article.

If the boot procedure announces a Controller Error, it will be necessary to setup the CMOS for a TYPE 0 (zero) drive. If booting from a SCSI drive, set the first CMOS drive to TYPE 0. If booting from any other type drive, set the second CMOS drive to TYPE 0. Newer BIOS will automatically sense SCSI drives and do not require user setup.

Software Installation
Caution: Most SCSI installation software will only install drivers for the SCSI devices that are present and functional. Be sure to complete all hardware installation before attempting driver installation. Repeat software installation each time a new SCSI device is added to the system.

Some SCSI host adapters require installation of drivers. Some host adapters only require drivers if more than two hard disks are used. CD-ROMS as well as some tape backups and scanners require drivers. Ideally, the host adapter manufacture has included a program such as Adaptec's EZSCSI for driver installation.

EZSCSI will install the drivers and SCSI utilities on the boot drive. As with hardware installation, a little planning and preparation will save a great deal of time. The SCSI utilities include a low-level SCSI formatter (SCSIFMT.EXE) and a SCSI version of FDISK (AFDISK.EXE) The use of these utilities may present a "chicken and egg" dilemma. For instance, if the utility to low-level format the disk is to be installed on the same disk that is to be low-level formatted, the format procedure could interfere with the formatting program. Some applications will run entirely in RAM but this is always risky. Fortunately, there is an easy solution.

EZSCSI can be run from either a floppy or hard drive. Sometimes it may be necessary to run EZSCSI to install the utilities on a floppy, perform the necessary low-level formatting, and then rerun EZSCSI to install the utilities and drivers on the hard disk. Unless there is information on the disks that must be preserved or your dealer has certified the disks do not need low-level formatting, it is a good idea to low-level format the hard disks. It is essential to format disks that have been previously used on other systems such as Macintosh or Novell. Warm up any disks before formatted by running that disk for at least one half hour.

Follow the low level format with FDISK for boot disks or AFDISK for logical SCSI hard disks. (If you are lost and don't know what a SCSI logical disk is, both FDISK and AFDISK will inform the user if they are mis-applied.) Complete the process by high level formatting all partitions on the disks. Use the command: "FORMAT C: /U /S" on the primary boot partition.

Caution: It will be necessary to complete driver installation before accessing logical SCSI disks. This is because AFDISK and FORMAT can not find the SCSI logical disks until the drivers are installed.

To install the drivers, type EZSCSI and hit ENTER twice to start the installation process. The program will show all the available SCSI devices. Verify that all installed SCSI devices are listed. EZSCSI will ask for the area to install the drivers and utilities and suggest, C:\SCSI. (Change this to your floppy drive if it is necessary to use the utilities on the boot disk.) The boot disk should contain all drivers, even if booting from a non-SCSI disk.

If necessary, complete installation with AFDISK and FORMAT for any logical SCSI disks. (The boot disk has already been formatted. Reformatting at this time will destroy the newly installed drivers.) EZSCSI will ask for a drive letter to assign the CD-ROM. Pick a letter that is not in use. Frequently a letter like "M:" is chosen to allow for future hard drives. (If CONFIG.SYS contains a "LAST DRIVE =" statement, it may require editing to conform to the CD-ROM.)

At this point all SCSI devices should be accessible. Tape or scanners will require separate application software. The CD-ROM will appear to all applications like a hard drive with three exceptions:

1) All files copied from the CD-ROM will have their READ ONLY file attribute set. This will prevent erasing or moving the files if they are copied to a hard disk. Consult the MSDOS APPLICATIONS manual or your favorite file utility to change file attributes.

2) Access to the CD-ROM will be slower than a fast hard disk. A few rare applications like BBS programs must be informed they are using a slow disk. Consult these specific applications for CD-ROM access.

3) Adaptec's CDPLAYER and other special utilities will allow running other types of non-MSDOS CD-ROMs, such as audio CDs. (This assumes the CD-ROM player is capable of handling such a CD. For instance, Kodak PHOTO CDs require multi-session players and full-motion video will require at least dual speed players.) Next month, SCSI hint and kinks, including how to make your own cables.


Page 12 had ads for GEP Buchmann and Just Computers! (www.justcomp.com).

Page 13 had a full-page ad for the Clark Development Company.

Page 14 had ads for IBBS West, the Automobile Network, Monterey Gaming System, and Lincoln's Cabin BBS.

Page 15 had a full-page ad for Mustang Software (www.mustang.com).

Page 16 had ads for the Olde Stuff and Eyes on the Skies (www.hooked.net/~tvs) BBSs.




ISDN on your BBS?

(By Dan Majhor, Motorola UDS)

Has ISDN's time arrived? Integrated Systems Digital Network is an international standard that has languished on the sidelines in the US because it was expensive to install and operate. As of January, in all of California there were only 12,000 ISDN phone lines. That will change in this decade, as Pacific Bell lowers its rates and deploys new phone switches throughout California's metropolitan areas (BABBA, March 1994). ISDN will be available to more than 95% of Californians by 1998.

Digital services will serve many complex business applications better than analog services, but will they work for BBSs? To test the market and spread the word, Pac Bell has set up an ISDN BBS.

As digital services become practical (and cost-effective) for general use, Sysops may find ISDN an attractive option for their BBSs. As the momentum for telecommuting grows, more companies will establish ISDN links to their private networks. Because ISDN is worldwide, it makes high-speed connections to remote locations faster and more reliable.

ISDN has been available to business for years, but the general public has not become acquainted with it. Integrated Systems means the integration of voice and data into one transmission line. Digital Network means the phone line carries only digital from end to end, unlike today's analog phone lines.

Analog data is only converted to digital at the telephone company's local office if it is to travel over digital trunk lines. For instance, your voice in a typical long-distance call would be digitized at the local office before it is fed into a channel bank or phone switch, to travel over digital long-distance lines. Your voice in an ISDN call, on the other hand, is digitized in your home before traveling over phone company wires.

ISDN sends data at speeds much higher than the modem-based services we commonly use today. A single ISDN channel can transfer data as fast as 64 kbps. Since ISDN lets you use more than one channel for a call, under the process called channel bonding, digital line speeds can reach 128 kbps or more.

Today's fastest analog modems establish a 28.8 kbps connection under ideal conditions. Using built-in compression, data rates can theoretically reach 115.2 kbps, but only for compressible files. Most files available from BBSs are already compressed. Compressed files don't transfer faster than the 28.8 kbps limit. Compression is important to telecommuters, though. Data streams can typically be compressed (before entering the ISDN line) to a 4:1 ratio, allowing data transfer rates to reach speeds of 256 kbps, and with channel bonding, 512 kbps.

ISDN in the Home
Basic Rate Interface (BRI) is the level of ISDN service aimed at home use. BRI is also known as 2B+D. BRI divides the ISDN connection into two bearer (B) channels to transfer data or voice, and one 'D' channel for call setup and tear down. Each B channel is typically 56 kbps (Pac Bell Centrex subscribers have B channels of 64 kbps). The D channel is 16 kbps. Amazingly, all three channels come to your home or office on only two wires.

ISDN's Country Cousin: Switched 56
Digital services have evolved since the days when you had to lease extra analog phone lines to get more bandwidth over the public switched telephone network (PSTN). At one end of the spectrum are high-capacity T-1 (1.5 Mbps) and T-3 (44.7 Mbps) services, which are so expensive that some subscribers sell portions of their T-1 bandwidth to others (this is called fractional T-1).

At the low end of the spectrum is a 56 kbps digital service called Switched 56. Switched 56 is commonly used to establish a live connection to the Internet. The name Switched 56 means that it can be connected to different endpoints in the network. Unlike ISDN, (which carries data, voice and control signals) Switched 56 lines carry only digital data. Under the new ISDN pricing, a Switched 56 line will cost about the same ($45/month plus use). As in conventional analog service, both Switched 56 and ISDN customers get their own phone number.

Today there are many more Switched 56 than ISDN subscribers, and ISDN products, such as Motorola UDS's TA120 terminal adapter (discussed later), accept calls from both services. The terminal adapter looks at calls coming in from the network, determines if the call is Switched 56 or ISDN, and then connects to it. Pac Bell's BBS uses this feature with their Motorola UDS terminal adapters to accept connections through Switched 56 service.

Pac Bell sells Switched 56 under the name SDS-IS, which comes in a two or four-wire configuration. The four-wire service is more common. Motorola produces connection units for both types of Switched 56 service. This box, called a CSU/DSU (Communications Service Unit/Data Service Unit) looks like old-styled modem, but works only over Switched 56 lines.

Business Solutions from Digital Services
Think of the possibilities. One ISDN 2B+D line into your home could connect your PC to the server in your office across town at 256 kbps (with precompression) on one B channel, while you simultaneously carry on a conversation with your boss over the other B channel.

Pac Bell's ISDN monthly rate for this service is less than $30 (if you subscribe to Centrex, $45/month otherwise). Similar to today's analog business services, the caller pays for connection time. Unlike analog services, even local ISDN calls are not toll free. In home use, expect to pay four cents for the first minute, and a penny per minute for calls to points within eight miles.

Businesses supporting telecommuters may need more capacity than BRI, and ISDN's modular, scalable nature makes it easy to expand the bandwidth. Designed for business, the Primary Rate Interface (PRI) includes 24 B channels, and each can be allocated as required. For instance, 10 channels could be used for 10 remote callers, and 14 channels could be bonded together to provide nearly 900 kbps bandwidth to a remote office LAN.

For both home and business, Pac Bell waives the substantial installation fee if you keep the service for two years, or prorates the fee if you don't.

Terminal Adapters
To connect your phone, modem, PC, or fax machine to ISDN service, you need a device called a terminal adapter, and another called an NT-1. The terminal adapter assembles data (including digitized voice) into digital data packets for transmission. A terminal adapter can fit into a PC, or reside as a stand-alone device. The terminal adapter has several ports, and requires electric power.

Terminal adapters feature V.120 rate adaptation. V.120 is an international protocol developed by the ITU (formerly CCITT), that allows asynchronous or synchronous data transfer over a high-speed data channel.

Let's say the PC on each end is a 386SX, and you believe 19.2 kbps is the top speed for the (DTE) serial port. The 56 kbps V.120 synchronous data connection adapts to the asynchronous 19.2 kbps data to the PC. On the Motorola UDS TA120 terminal adapter, this is easily configured from a keypad and LCD on the front panel. Terminal adapters are currently costly, priced between $500 and $2000.

NT-1
The Network Termination-1 adapter (NT-1) protects the ISDN network from faulty equipment by isolating the network from the terminal adapter and all phone equipment connected to it. Using time division multiplexing, the NT-1 combines the two B and D channels from the terminal adapter onto just one pair of wires. NT-1 prices range from $150 to $400, which includes the power supply. In a power outage you will have no phone service, unless you can generate power for both the terminal adapter and NT-1.

When combined with a terminal adapter, the NT-1 device becomes the gateway to ISDN in each home or office. (In media talk, these are the on-ramps to the information highway.) The terminal adapter feeds phone signals directly into the NT-1, which connects to phone company wiring.

You won't have to scrap your old phone equipment after installing ISDN. A jack on the terminal adapter, called POTS (Plain Old Telephone Service), functions like today's phone jack. The POTS jack lets you operate a conventional modem or analog phone over digital lines.


Pac Bell's ISDN BBS
Pac Bell promotes ISDN and Switched 56 services from a BBS in Pleasanton, CA. The BBS connects to the network through analog modems (Hayes 14400) and ISDN terminal adapters (Motorola UDS TA120). The BBS has 16 lines and two phone numbers - one analog, and one digital. To the BBS software and serial card, the ISDN terminal adapter appears to be a high-speed modem.

In the block diagram there are two pair of wires coming out of the Motorola UDS TA120 2B+D terminal adapter. with one voice port (POTS) and one data port (RS232, V.35, or RS530). One pair of wires goes from the POTS jack to a 14.4 kbps modem, then into the PC. The other pair carries digital data directly to the serial card in the PC. Both the digital and analog connections operate at the same time over the same pair of wires, with no degradation of throughput. For analog old-timers, this is pretty amazing stuff.

Set up an ISDN BBS?
It may be a few years before BBSs yank out their analog phone lines, but some Sysops could benefit immediately by using ISDN links to other systems and networks. For example, Fidonet trunk carriers could use ISDN to speed their nightly mail haul and file transfer. Certainly business will soon embrace ISDN as the preferred telecommuting/remote office connector.

If you want to expand your BBS to include ISDN, here are some recommendations:

1) Familiarize yourself with the technology. Visit their web site.

2) Call Pac Bell's voice number to verify the price and availability of digital service at your location. You may have Switched 56 service available, or ISDN service, or both. Discuss pros and cons with your applications engineer. Remember, you do not have to forfeit your investment in analog phone equipment when you add digital service.

3) Once you've done your homework and are ready to proceed, call me (This article came out April 1994, so don't call about this.) at Motorola UDS for product information. I will be glad to answer any questions and can assist with every step of your ISDN implementation.

4) Cost out the BBS hardware and software: plan on at least a fast 486 PC or 68040 Macintosh, and a fast (16550-enhanced) serial board. The Pac Bell BBS uses a Digiboard to handle multiple phone lines. With fast hardware, most BBS software packages easily handle the data transfer rates of ISDN. The Pac Bell BBS uses eSoft's TBBS software. When you are ready to buy a terminal adapter, contact me.

5) Call your local Pacific Bell account executive to order ISDN service. Coordinate your line installation with your sales rep and applications engineer to install your ISDN hardware.

What else can I do with ISDN?
Pac Bell's BBS is one application of ISDN. There are businesses using it for Point of Sale, telecommuting for CAD, engineering, and many other applications. There is a catalog of applications available from the National ISDN Users' Forum. In states that allow Caller ID, ISDN can be used for Automatic Call Distribution. To learn more, contact Pacific Bell (www.pacbell.com) and Motorola (http://mot-sps.com/solutions/isdn/apps/nt1.html).


Page 17 had an ad for the Black Rose BBS.

Page 18 had ads for DC-to-Light, and Mookie's Place BBS (mookie.relay.net).

Pages 19 though 26 had detailed listings of Bay Area BBSs - and had ads for MarketScope (www.fastgain.com), and the Anathema Downs, PRiME MERiDiAN, and Bust Out BBSs.




File Extension Definitions

(By Alex Riggs)

While you were looking at a file listing on a given BBS, you may have wondered about the types of files you can download. Or, perhaps you already downloaded a compressed file, and unarchived it only to see a bunch of files with strange names.

Many file types are distinguished by their name extensions. File extensions can help you determine the file type, even if the filename is cryptic. On DOS-based computers, file names are limited to eight characters, with an extension of three characters (i.e., FILENAME.EXT).

File extensions have become associated with specific applications and manufacturers, and are too numerous to list here. This article should give you an idea of what types of files you are likely to see on a BBS.

Compressed files
A compressed file is one or more files that have been processed by another program to save disk space, add protection from virus infection, and reduce modem transfer time. A 100K text file can be compressed into a 10K file, saving you download time and making space on the BBS for other files.

File compression keeps files organized on a BBS system. For example, a program that needs 10 files to function could be compressed into one single filename. This lets a caller to download just one file to get all ten.

Compressed files usually cannot be run or used as they are. Another program must be used to convert them to their original form. Most BBSs make available several of the programs necessary to compress and decompress files you find on the BBS. Here are some common file extensions for compressed files:

File, Extract Thyself!
There is one very special type of compressed file called a Self-Extracting Archive. These compressed files usually have an extension of .COM or .EXE. On the Mac, the most popular self-extracting file format is .SEA. These programs, when run, will decompress themselves without the help of another file compression program.

Self-extracting archives place their decompressed files in directories and subdirectories created by the archive's creator. It's a good idea to put these files (or any compressed file) inside a directory of their own before decompressing the files.


Graphic Files: These files are pictures, movies, or animations. These files usually need another program to run or view them. Again, most BBSs have the programs available that are needed to view these files. Some more common names of animation and picture files:

Many graphic file types compress well. Consequently, you will often find these files in a compressed format. Exceptions are .GIF and JPEG files, which are already highly compressed.

Sound files:

Text files: These are almost always plain ASCII text; this type of text is readable by editors and word-processors, no matter which computer platforms you use.

Both Text and Sound file types compress well, so they are often stored in a compressed format.


Page 27 had ads for the Party Wherehouse, Burn this Flag, Party Line (www.partyline.com), Roadkill Grill, and UFO BBSs.

Page 28 had ads for Tactical Alternatives, Courtesy Auto Service, Arsenal Computer, and the Slaygor's Domain and InfoDude Communications BBSs.

Page 29 had a full-page ad for a full-page ad for Galacticomm (www.galacticomm.com)

Page 30 (back cover) had a full-page ad for TeleText Communications.




End of Issue 14. Go back, or to Issue 15, or to Mark's home page.