Wether new or old, there are amazing similarities among the PC-clone computers, to the point where almost all parts are interchangeable. Not infrequently a PC will be constructed from the components from a dozen different junk machines and then brought to life -- a true Frankenstein.

It is this fact which resulted in the wild popularization of the PC. When IBM released the specifications to the public in the early 80's, anyone who wanted to could manufacture a machine and use peripherals and parts built by another manufacturer. The result has been a wild competitive proliferation in parts as well as software.

When someone offers a used computer to you, take it. The XT, AT, and PC computers were built to commercial specifications. To realize what that means, you could check an Allied or Newark parts catalog of years ago. Almost any electronic component was available in four different grades, with a ten-fold increase in price across the four columns.

Some things can be pre-tested for their expected life, and their manufacture can be controlled. You pay for what the manufacturer thinks the life will be. The end result is a grading of parts into some four classifications, as follows.

• consumer grade -- these have an expected life of 7 years, and you better not drop them off a table.

• commercial grade -- these have an expected life of 15 years, and will be expected to operate under modest environmental conditions like an office.

• industrial grade -- have an expected life of 20 years, and could be operated under severe conditions, high heat, corrosive atmospheres.

• military grade -- have an expected life of 30 years, and will operate under extreme conditions, severe cold, jungle heat, and will withstand explosions and shock.

The desktop computers built by IBM in the early 80's were built to commercial specifications, as they should have been, since the early users were paying up to $3000 for machines.

This trend has continued mostly, but there is no guarantee that products expected to be made absolete by newer hardware within four years, as is the case today in the late 90's, will still be built to these specifications.

It might be suggested that older puters, if they are still running, might be a better bet, for many machines from the late 80s and early 90's will still be operating in the year 2010 or 2020.

Certainly just about anything on an older computer can be replaced or exchanged with newer parts, or with other old parts from yet another surplus machine. So if someone offers you a computer, take it. If nothing else, you could use it as a terminal to you main computer, or use it as a FAX machine, or just play tic-tac-toe on it.

A few machines are not worth it (although parts and cases can be used), like all the early XT machines, which operated at a deafening speed of 1 megahertz. And others, below.

• The CMOS of the XT is set with 8 or 16 dip switches, which include settings for the amount of memory, which can vary from 64K to a full meg. If you do not have access to diagrams for these switches, it will drive you nuts trying to experiment. The XT takes like minutes to time out after each failed boot up.

• The keyboards on XTs are known as type "83" -- the plug is the same as on most later keyboards, but a "101" keyboard will not work correctly or at all.

• The monitors (MDA) were great, though, although they weigh a ton, and most of the video cards are slow as molasses. The type face is clean and easy to read, if green on black meets your idea of "easy on the eyes." Many of these monitors will outlast current VGA monitors.

• The three inch high floppy drives, rated at 360 KB, can be used as long as the contacts on the arm haven't been bent or worn through. The 360KB floppy was the standard computer-to-computer interface for 15 years.

• The XT came with bays for two hard drives, and two floppy drives. That will be a 10 megabyte hard drive. Take the hard drive apart to see what it looks like on the inside, or use it to keep doors open.

• The cases are sturdy, well built, and could be used for other mother boards if it weren't for the fact that the slot to slot spacing changed with the introduction of the AT. But you can change the slots (remove the spacers and drill new mounting holes). The power supply can be used for anything except a full complement of add-on cards.

• The XT and AT computers which sport an 085 or 086 processor. They are just too slow. The IBM BIOS ROM includes a full BASIC library, however, which kicks in when a command processor cannot be found.

• Any mother board which only has 8-bit ISA slots (all the XTs). Require at least a few 16-bit slots to keep the board; better yet a board which also has one or more VLB slots.

• Mostly the whole of the PS2 series by IBM. Nothing fits these machines. The keyboard plugs look like they will fit modern computers, but you have to break off a plastic stub in the DIN plug, and then live (I think) without the F11 and F12 keys.

• The IBM PCjr series (and the Tandy 1000 clone). They have plastic cases, which means that you cannot set the monitor on top, for the monitor's magnetic field will screw up the operation of the processor.

  But don't entirely discount these either. Some come with additional memory, and LPT ports, and the machines include programmable sound chips which are absolutely great (if you can find sources for the required code). They thus provide sound out, as well as composite video.

  The monitors and keyboards fit nothing else, however, although the monitor plugs can be rewired for use as EGA monitors. The monitors include speakers.

• Any of the computers based on the Z80 processor (like the Sinclair), or which still use a CP/M operating system.

  A note on the Sinclair: This tiny puter has a UHF output, which allows it to be plugged into a TV set. There's your monitor, and that is all you get. But it has a built in BASIC instruction set, all entered with single keystrokes. Thus if you can handle BASIC, it can still be used to title videos, including tricks like scrolling.

  The Sinclair BASIC instruction set included a few unusual string handling parameters, such that you could create a full-blown word processing program in a some 30 instructions. The drawback was the lack of any way to save such a program, and the lack of a decent printer. Cute though!

• Avoid machines build completely on a bus, that is, where the processor is just another plug-in board, like the Kapro, and some Zenith boxes.

• Immediately scrap absolutely non-standard machines like the Epson computers.

• The whole Tandy TRS series, or whatever, up to the NE series which used a 0286 processor, and which become a PC clone in the process. Too bad, for at one time in the late 80's there were more Tandy computers in the world than any other machine.

• If it is a 286, keep it, or at least look at it. Some of these will run at 20 megahertz. That is certainly faster than you can type.

• If it is a 386, test it. Some 386 are very fast, but many are crippled children from the Intel industry, and run at speeds slower than many 286 processors. If the 386 is soldered in, it is likely to be an industry reject sold for its name rather than its capabilities.

• Many 486 processors are OK by today's standards, and a few run faster than early 586 (Pentium) processors.

Other things you can say goodbye to include all the MFM hard drive controllers (HDC) of the past. These are the ones with two flat cables to the drives. Too bad, for these are an estimated 60 million MFM hard drives out there, ranging from 10 megs up to 150 meg.

Keep IDE HDC cards; you might want to use it for wiring in a CDROM, a backup tape, or a Zip drive.

Most all the 83, 84, and 102 keyboards are useless if you are used to a 101 keyboard. Plugs have remained the same for years, although today the smaller DIN plug is used. Converters between plugs can be found, though.

Keep any 101 keyboard which feels good (like: a nice click, big BS key); you can use it anywhere.

Scrap any CGA video cards and monitors. CGA is actually so bad that a half hour of reading from the monitor will give you a headache. EGA is fine for any applications which don't do high-end graphics.

TTL monitors (which will adapt to a variety of graphics cards) are OK also, if you can get them to work.

Keep Hercules monitors and cards, they are reasonably fast, do very fine characters (like an MDA), and will do graphics -- in black and white, of course, or more likely in amber.

Generally you could get rid of any 8-bit cards, as they will be slower than 16-bit cards, with the exception of COM port extension cards (I/O cards) which are always 8-bit. And you will always need another COM port or LPT for some machine.

Net cards are not worth keeping. They are amazingly difficult to set up because of the guessing involved with unidentified jumpers, and the missing (software) drivers. The exception are the NE2000 clones.

The 5 1/4" floppy drives are now officially on their way out, although there are still millions of 5 1/4" floppies out there. The earliest floppy drives in general use were 360 KB, the last write 1.2 Meg. Most any 1.2 Meg floppy drive will also read any other value, including the Tandy 720KB.

The 3 1/2" floppy drives today read 1.14 Meg as a standard, and most will also read 720 KB floppies, but no PC will read the MAC 800KB floppies, which are hardware dependent.

A 800K MAC floppy drive cannot be used on a PC, for the HDC will not recognize the strange layout. And floppies formatted by early MAC 800 FDs will magnetize the read heads of 1.44 FDs, that is, they will destroy them.

You will run into "timer" cards, sound cards, modem cards, and add-on memory cards (16-bit slots, and a meg each). At times you can get the meg-cards to work on some 286's where their lack of speed doesn't matter.

Hand held or flat bed scanners, keypads, graphical device, etc, all require software, and you will be out of luck without it. Search the hard drives before you wipe the drive.

Most any printer can be used on any other system, even a MAC, although you may need a serial to parallel conversion adapter. MAC serial printers can be used on PCs.

Save all the cord and cables, as spares, and in order to avoid having to pay good money for new ones. Save all the screws; three sizes are used for almost all computers.

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