Technology and retro computer blog

Altair 8800 S100 boards

MITS 8800 CPU board - the cpu card works fine but on close inspection assembly is very rough.  Many of the traces have been re-soldered, and it looks like quite a few hacks have been done to this card.  The traces are prone to blackening.  And at one stage the machine wasn't working properly, it wouldn't respond to the front panel when turned on.  On removing the cpu card I noticed that the contacts on the card's edge connector were very blackened, so I lightly rubbed them with some steel wool, (not recommended, use an eraser instead).  This fixed the responsiveness problem.  Obviously the contacts weren't making a very good connection.  A simple fix which I was pretty happy with.  This particular cpu board has an Intel 8080A processor installed, date on the chip is 1974.


California Computer Systems 64k dynamic ram board - since getting the Altair  I've purchased a couple of memory cards, the first was a TDL16k which unfortunately had stuck bits.  I also purchased this CCS 64k which seems to be ok.  Being a 64k card it provides the Altair the maximum amount of memory.  It's also a fairly late s100 card dated 1980.  There's a set of bank select jumpers in the top left corner which allows any 16k block of memory to be disabled.  I've tested the CCS64k using the Altair Rom monitor by John Garza and it gives it a clean bill of health.


Cromemco 8k Bytesaver - The 8k Bytesaver is an 8k prom card that can read and write 2708 eproms.  It has capacity for eight 2708 eprom chips (1k each).  These cards were developed by Cromemco primarily for use in an Altair.  I have two 8k Bytesavers that I can use in my Altair.  The 8k Bytesaver has enough capacity to store 8k Basic on seven eproms and still have enough space to store a 1k monitor program for loading software and examining memory.  I've also created socket adapters for using more modern 2716 eproms and 2816 eeproms with the Bytesaver (see post).  Using the Bytesaver I can load 8k Basic into the Altair with a few switches after power up.


Solid State Music IO4 - the serial card in the Altair is an SSMIO4.  This is a very rare s100 serial card because it can be configured for use in different s100 machines.  Importantly for the Altair it can be configured exactly like the MITS 88-2SIO to run MITS software.  See my guide about configuring the SSM IO4 for the Altair.


MITS 88-S4K memory card - I have two of the MITS 88-S4K cards.  One seems to work fine, but the other is a little flakey.  I've managed to do a simple memory test on both cards using the Bytemover program.  The Bytemover program moves a 1k chunk of memory from any 1k location withing an 8k boundary.  Using this you can copy the program from one 1k chunk to another and then run that copy.  If the copy moves the memory successfully it's a good test of the memory in that location.


MITS 88-2SIO Serial Card - the MITS 88-2SIO pictured above is configured for RS232 serial interfacing.  The wirewrap hack that you see on the left, sets serial ports 0 and 1 to addresses 20 and 22 (octal).  This is the standard terminal I/O address for 88-2SIO to use MITS software like Basic.


MITS disk board 1 rev. 0 X3 - this is one of the two boards that makes up the MITS floppy disk controller.


MITS disk board 2 rev. 0 X2 - this is the other board in the disk controller set


An adapter for replacing 2708 eproms with 2716 eproms

2708 eproms were used in early computer equipment and arcade machines, but unlike more modern eproms they require a triple voltage for programming.  Because of this odd voltage requirement many current eprom programmers don't support them.   2716 eproms are later model eproms which have a very close pin configuration to the 2708's, but are supported by many current eprom programmers.  A simple three pin modification can be made to the 2716's to make them usable as 2708's.  2716's have a 2k capacity, as opposed to 2708's which are 1k, so you only use half of the capacity, but as they are readily available and fairly cheap, this is not a big issue.  2816 electronically erasable eeproms can also be used instead of 2716's.  The 2816's are pin compatible with 2716's but are electronically erasable rather than UV erasable, so they are even easier to work with.


To avoid making changes to the original 2708 equipment and the 2716 chips, a 24 pin socket adapter can be made to fit between the original 2708 socket and the 2716 chip.  This allows the 2716 to be programmed on a modern programmer and then placed into the adapter socket for use as a 2708.  To create the adapter, I use two 24 pin sockets, the upper socket contains the wire connections and the lower socket has pins 18,19 and 21 removed.  See pictures below.


To create the adapter, the following connections need to be made in the upper socket:


  • Connect pin 21 to pin 24
  • Connect pin 18 to pin 20
  • Connect pin 19 to pin 12  *


* Pin 19 is connected to pin 12 which uses the first 1k in the 2716.  Alternatively pin 19 can be connected to pin 24 to use the second 1k in the 2716.


Then in the lower socket pins 18, 19 and 21 need to be removed.  See pictures below.


Bottom view of upper socket, bottom far left pin is pin 12, top far right pin is pin 24



Completed adapter showing bottom socket with pins 18, 19 and 21 (left to right), removed



Top view of socket adapter, top far right pin is pin 1, bottom far right pin is pin 24



This diagram shows the pin assignments for both the 2708 and 2716 and the required wiring connections and pin removals to enable the 2716's to be used in place of 2708's



 Click on image for larger version
Here are the adapters in use in an eprom card installed in the Altair8800.  The Cromemco 8k Bytesaver card has capacity for eight 2708 eproms.  2816 Xicor eeproms have been used in all eight slots using the adapters.  Chip zero (far right), contains a 1k rom monitor program and chips 1 to 7 contain MITS 8K Basic.  Using the monitor program 8k Basic can be transferred into ram almost instantly with a simple command from the terminal.






Mini vMac

Mini vMac is a great emulator of early 68k Macintosh computers.  See Mini vMac site for more details.  I've prepared two downloads of Mini vMac.  One emulates a Macintosh 512k, running System 2.0 from a 400k floppy and the other emulates a Macintosh Plus running system 6.0.8 with a 40mb hard drive.  The downloads below include rom images and disk images with software already installed.  So they are ready to run.  These downloads contain the Windows executable of Mini vMac, but if you're on another platform just replace the executables with the ones you require from the Mini vMac website.


The distributions below have the ImportFI and ExportFI utilities already installed on each disk.  These utilities allow you to easily import and export files between the emulator and the host system without running any other software such as HVF Explorer.  They also have BinHex 5.0 and PackIt archiving utilities which allow you to easily package files for transfer to a physical Macintosh via serial cable.  See my guide: Serial cable file transfer, pc to Macintosh 128k/512k .


Follow this link for the Macintosh 128k / 512k disk image archive


Serial cable file transfer, pc to Macintosh 128k/512k

Getting software onto floppy disks for old Macintosh computers, such as the original 128k and 512k can be difficult.  The problem is the floppy disk standard they used is no longer supported on any modern machine.  These early Macs used standard double density 3.5" disks, but they were formatted with a 400k and later 800k floppy format.  This standard is no longer used by modern computers.  The old Macs are also difficult to connect to networks, especially pre-SCSI Macs like the 128k and 512k machines.  They had serial ports but a problem with serial transfer is that they used the RS422 standard, not the widely used RS232 standard that most computers, like the Apple II, use.


Many enthusiasts of these old Macs use a bridge computer to transfer software to disk.  There were several Macintosh computers produced during the late 1980's and 1990's that supported both the old 400k and 800k formats and also the more widely used PC, 1.44mb high density format.  The drive in these machines is called the Superdrive.  Some models that have the Superdrive are the Macintosh SE/FDHD, the SE/30 and the Powerbook 520c and 540c laptops.  There are also others.  A bridge machine offers a great solution especially if that machine has ethernet capabilities.  While a bridge machine is great, if you don't have one, the simple solution is to use a serial cable.


USB to serial adapter cables are cheap and readily available and they will add a serial port to your modern computer, but they are wired for RS232 standard serial communications not RS422 like the old Macs.  The solution is to create an RS422 to RS232 adapter cable.  Then you simply connect this cable between the Mac and the USB to serial cable.  See description below for how to create this cable.


Another issue with old Macintosh computers is that they're unusable without a boot disk.  Unlike the Apple II which has inbuilt Basic in ROM.  So unfortunately if you purchase one without any disks you can't create the disk to use it, like you can with the Apple II using something like ADT Pro.  You need to get your hands on a bootable 400k floppy disk.  What's more to transfer software to the old Mac via serial you need a couple of important utilities on that boot disk.  The required software is described below.


Macterminal transfer
Xmodem transfer from Windows 7 pc running TeraTerm to Mac 512k running MacTerminal 1.1


Creating the serial cable

The following table shows the pin assignment of the Mac 128k/512k de-9 serial connector (RS422 standard).  The second table shows the 5 pin adapter cable required to convert RS422 to RS232.


Macintosh 128k/512k RS422 serial connector
Pin Description
1 gnd
2 +5v
3 SG
4 TxD+
5 TxD-
6 +12v(HSKo)
8 RxD+
9 RxD-


The table below describes the connections required to create an RS422 to RS232 adapter cable.  The connector at the back of the Mac 128k/512k is a de-9 female connector.  And the connector on most USB to serial adapters is usually a de-9 male connector.  Therefore the cable below has a male de-9 connector at the Mac end and a female de-9 connector at the other end.


Macintosh 128k/512k RS422 to RS232 serial adapter cable
Mac de-9 Male PC RS232 de-9 Female PC Signal Name
7 7 CTS
6 8 RTS
9 3 TXD
1 5 GND
5 2 RXD



Required software

In order to transfer files to the Mac, you need a few utilities: MacTerminal 1.1, BixHex 5 and/or Packit 1.0.  The terminal program is required to do the file transfer and BinHex or Packit is required to preserve the resource fork on the old Macintosh files. BinHex 5 will only archive a single file, whereas Packit allows you to put multiple files into an archive.  But the great thing about these utilities is that both will open any file once transferred, they don't rely on the file type.  The following table shows the software on a 400k boot disk that will allow you to boot the  Mac and do file transfers.  Even with all required software this disk still has around 180k free, so you can transfer files directly to it.  The system components for this disk have been stripped right down of unnecessary fonts and desk accessories to maximize free space on the disk.


Macintosh 400k boot disk for serial transfers
Utility size
System 2.0 50k
Finder 4.1 47K
MacTerminal 1.1 98k
BixHex 5 8k
Packit 1.0 9k


Download the disk image for this disk here: (176.29 kb)



By booting an old Mac with this disk, you can transfer files in a BixHex or Packit archive across the serial link using XModem in MacTerminal.  If the files will fit, you can transfer them directly to the boot disk.  If the files are too large for the boot disk, MacTerminal allows you to eject the boot disk prior to receiving a file, so you can write the receive file to a blank disk.  This gives you the ability to transfer larger files.  This process is made much easier if you have and external floppy drive, as you can extract the archived files to the external drive.  If you have a single drive you can still do it but you the process will require disk swaps.

This is a complete solution for transferring files from an internet computer directly to a Mac 128k/512k, without using a bridge machine.

See also my Macintosh 128k/512k disk image archive

Apple II Plus

This is my Apple II Plus, it has 64k, 48k on the motherboard and a 16k Microsoft Ramcard.  It also has the Microsoft z80 Softcard installed so it can run CP/M.  The monitor is an Applecolor composite monitor IIe.



 Apple II specifications
Manufacturer    Apple
Release date April 1977
CPU MOS 6502, 8 bit
Speed 1 MHz
Ram 4k to 48k
Rom 12k
Storage Cassette, optional 5.25" 140k floppy disk  
Expansion 8 slots
Ports Composite video out, cassette
OS Rom Basic, Apple DOS


Cards installed:

  • Microsoft Ramcard
  • Microsoft Softcard
  • Super Serial Card II
  • Disk controller II



Macintosh 128k

This is a Macintosh 128k 240v international model.  The model number is: "M0001P", unlike the U.S. 110v model which is M0001.  The "P" indicates the 240v international model.  I purchased this computer as a non-working machine with the original 128k logic board in it.  It also came with an external 400k drive.

Macintosh 128k Macintosh 128k Macintosh 128k


  Macintosh 128k specifications
Manufacturer    Apple
Release date 24th January 1984
CPU Motorola 6800, 32 bit
Speed 8 MHz
Ram 128k
Rom 64k
Storage 3.5" 400k internal floppy disk
Expansion n/a
Ports RS422 serial, mouse, floppy disk, audio
OS Macintosh System 1


On initial power up, it displayed an error code, but soon after that it would just power on with several beeps which indicated a defective logic board.  I replaced the logic board with a working 512k logic board, so in effect this is a Mac 512k. The logic board for the 512k has exactly the same connectors on the back so it's a simple drop in replacement for the 128k logic board.


I've also replaced the power sweep, the original burnt out shortly after I got it.  I replaced it with a 240v Mac Plus power sweep.  The power sweep for the Mac Plus is backwards compatible with all previous compact Mac models.


Both the internal and external 400k drives would not accept disks when I first tried them out.  So I removed the disk insert/eject mechanism from each drive.  On inspection both were jammed stuck from years  of sitting around.  I removed all the old muck off the mechanisms and re-lubricated them, and once back in the drives both drives worked fine.


Macintosh 128k Macintosh 128k Macintosh 128k Macintosh 128k Macintosh 128k back


Classic_Mac_Repair.pdf (1.06 mb)

This guide contains some invaluable information about repairs to common problem with classic Macs.

S100 boards that I've sold

When I first bought my Altair it came with many cards that I determined I just couldn't use.  Here's a description of the cards which I sold.

MITS 1K static memory
MITS 4K dynamic ram
Altair Basic & Programming package

 MITS 1K static memory
This is the original MITS 1k static ram board that came with the Altair.  This one was functional as far as I could tell from the limited front panel testing that I did.  Considering the small capacity of the ram board I decided to sell it.  I kind of regret this as the board was a great collectable, but in reality not much use for anything.


MITS 4K dynamic ram
My machine came with two of these early MITS dynamic 4k ram boards.  But by the original owners admission, both were a bit flakey.  And in my early testing I couldn't get them to work properly.  All of the MITS cards in my Altair came with the original documentation which is a huge bonus as this stuff is very hard to get hold of.


MITS 88-PIO parallel board
My machine came with a MITS parallel card.  But when I asked the owner about this card he told me that he never managed to use it for anything.  These parallel card potentially could be connected to a parallel terminal or printer.  At the time I didin't have enough knowledge of the Altair to do anything with this card.


MITS 88-ACR serial cassette board
My machine had a cassette interface board.  This board could potentially be used to load Basic and save Basic programs written by the user.  In the early stages of owning the Altair I didn't even have a working terminal so this card wasn't much use.


Altair 8k Basic v3.2 and Altair Programming Package #1 3.2 on casette
There were some copies of 8k Basic and the Altair programming package on cassette tape.  You can also see a printout of 8k Basic v3.2 actually running.  By the looks of it, the Altair must have had around 14k of working memory as the bytes free displayed on the printout is around 6k.

DEC VT100 Terminal

I've got an original Digital Equipment Corp (DEC), VT 100 terminal.  The terminal is ideal for use with the Altair as it comes from the same era.  I'm not sure exactly when this particular terminal was made but the VT100 was produced between 1978 and 1983.

 6th July 2013
Recently the VT100's screen went blank, and after some investigation I determined the flyback transformer had gone bad.  There's a small fuse on the video board that had also blown.  I replaced the flyback transformer and fuse, and now the terminal is up and running again.



  Digital VT100 specifications
Manufacturer    Digital Equipment Corporation (DEC)
Release date August 1978
CPU Intel 8085
Ports RS232 serial, composite out & in


    VT100 User Guide   VT100 Technical Manual


VT100 Animations

Send the contents of these text files to the terminal to display it's graphics capabilities.

vt100_merry_xmas_animation.txt (8.92 kb)

vt100_train_animation.txt (15.75 kb)

vt100_xmas_animation.txt (27.70 kb)

IBM 5150 PC

This is my IBM 5150 PC.  The 5150 was the first model of IBM pc, first released in 1981.  This one is dated 1983 and has the dual 360k 5.25" floppy drives.  The first model 5150 had 160k 5.25" floppy drives.  I bought the computer untested, I gave it a good clean and fired it up, everything worked.  I've installed an XT IDE compact flash hard disk card in the machine.  The CF card I've used is 32 meg which is plenty for a machine this age.  The power supply in the 5150 is not powerful enough to power a typical hard disk from this era.  The first hard disk model IBM PC was the XT which had a bigger power supply.


1983 IBM 5150 with PS2 vga monitor and Microsoft Green Eye mouse Microsoft Green Eye mouse dated 1985


  IBM 5150 PC specifications

Manufacturer    IBM
Release date 12th August 1981
CPU Intel 8088, 16 bit
Speed 4.77MHz
Ram 16k to 256k
Rom 40k
Storage two 5.25" 160k internal floppy disks  
Expansion 4 expansion slots
Ports Cassette, optional serial, parallel
OS IBM Rom Basic, PCDos 1.0



The mouse attached to the 5150 is the original Microsoft "Green Eye" mouse.  It was introduced with the first version of Windows in 1985.  This particular one has a DB25 connector and works with the generic CuteMouse serial mouse driver.


I've installed MS DOS 6.22 as the base operating system and I've also installed Windows 2.0.  I've also got some DOS games installed like Galaxian and Pacman.  The software gives you a good idea of what this pc was capable. of.


This machine has a Video 7 VEGA graphics card installed which has both 15 pin VGA and 9 pin CGA/EGA outputs.  The card can be jumpered from the rear of the machine to switch between the two outputs.


Microsoft Green Eye mouse top


XTIDE card and compact flash hard drive

In the last two slots seen above is a compact flash IDE card (left), attached to the the XTIDE card (right).  This allows the IBM5150 to use more modern IDE hard drives.  As my machine has the original 60 watt power supply I believe it wouldn't be enough to run an original IBM hard drive.  This is a good solution as the comact flash card is very low power and it can be removed and connected to a modern pc for copying software over to the 5150.  It also allows the 5150 to return to it's original dual floppy and no hard drive configuration simply by removing these two cards.


MITS Altair 8800

The MITS Altair 8800 is widely accepted as the first commercially successful personal computer.  It was produced by a company called MITS and was developed by H. Ed. Roberts.  The Altair was first publicly released in 1975.  It was also the computer that launched Microsoft, their first product was a Basic interpreter for the Altair called Altair Basic.

MITS Altair 8800 and 88-DCDD MITS Altair 8800 card view


 Altair 8800 specifications
  Manufacturer      MITS
  Release date   January 1975
  CPU   Intel 8080, 8 bit
  Speed   2MHz
  Ram   256 bytes to 64k
  Rom   Optional via s100 board
  Storage   Optional 8" floppy disk, paper tape, cassette tape   
  Expansion   s100 slots

  Optional: serial, parallel



I purchased this Altair 8800 in July 2011 from someone in Canada.  He was the original owner and builder and purchased it in 1975.  It came with many original MITS s100 cards and all the original documentation.  The serial number on the machine indicates it was a kit because the last letter is a "K" opposed to an "A" which meant factory assembled.  The serial number also indicates that this machine was number 1,919.  This means it was reasonably low in the production run.  From what I've read there were around 10,000 Altairs produced.

It took me almost two years and many hours of research to get the machine to a usable state where it could load software and was usable through a terminal.  Part of the problem was that the machine didn't have an RS232 capable serial card, and they aren't too easy to come by.  After a lot of effort I'm glad to say that I now have two working serial cards, a MITS 88-2SIO and a Solid State Music IO4.  The SSM IO4 is configured just like a MITS 88-2SIO and works just the same.   See my post below for full details.

Want your own Altair 8800?

Original Altairs are still available although they're becoming increasingly rare and expensive as time goes by.  The primary source for finding original Altairs and components  is U.S ebay.  But there's also some easier and cheaper alternatives for experiencing an Altair.  There's a number of software emulators freely available and there's also hardware replicas available at a reasonable cost.

Altair 8800 Clone

My pick for the best hardware replica is the Altair Clone developed by Mike Douglas.  The Altair Clone reproduces the external functionality and look of the original machine but on the inside uses modern hardware.  In fact if you look inside an Altair Clone it appears empty.  The entire functionality of the original machine has been recreated using some tiny chips on the front panel board.  The Clone is a fully loaded Altair with inbuilt capabilities for PROM and floppy disk emulation and also has an optional cassette tape interface.  There's a great library of Altair Clone demonstration videos on YouTube.  These videos are of great use to all Altair owners.  Mike's technical knowledge of the Altair is second to none and he provides excellent service and support.  The Altair Clone comes assembled or in kit form, but both are the same price.  Mike is also the developer of the FDC+, a disk emulator for original Altairs.

Altair 8800 micro

The Altair 8800micro is a hardware replica of the Altair that has front panel switches and lights.  It's smaller than the original machine and uses modern hardware on the inside.  It has various optional components which adds serial port and disk functionality.  It is priced cheaper than the Altair Clone and comes in a kit or assembled.

Altair 8800 Kit

The Altair Kit is a functional and cosmetic replica of the original Altair developed by Grant Stockly, it was available in kit form only.  It replicates the internal hardware of the original machine so closely that components from the Kit can be interchanged with the original Altair and vice-versa.  The case used for the Kit is the original case made by Optima.  The Kit is the closest replica of the original Altair ever made but unfortunately it hasn't been available for many years.  The website is still there and there's been recent rumors of Grant starting up production of the kit again.

SIMH Altair Emulator

A very good Altair software emulator.  It has many configuration options for different hardware and settings.

Altair32 Emulator

A great software emulator that captures the look of the original Altair in it's interface.  There are options for loading PROMS, disks and tapes and configuring memory.

Click here to see my other Altair related posts.

Altair 8800b from Visual Basic 5.0 intro 1997