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Tom W. Durston's Custom Altair

MITS Engineering Machine, born 4 Mar 75

(Bill Yates, MITS Head Engineer)

Tom W. Durston

The Altair 8800 mainframe shown here was used by me at MITS from 1975 though 1978 to develop and test Altair PCB designs and applications.

This unit has a unique engineering history at MITS, as I used it to develop the FDD controller, the FDD system, 88-ACR, 4K static board, and evaluate hundreds of enhancements, many of which were implemented in the 8800B.


This unit was also used for the very first test and debug of Bill Gates and Paul Allen's Altair Disk Basic and OS.  I couldn't go on Christmas Vacation until we found a way for software to check the disk 'data ready' flag and then read a data byte from the disk every 32 microseconds.  The Altair CPU wasn't fast enough to keep up with the disk data rate. We worked on a solution 24 hours per day until a fix was found. The ingenious solution was to input two bytes of data for each test of the  'disk data ready' flag.  I had Christmas a week late that year.


Tom Durston

MITS Product Development Engineer

April 27, 2017

Tom Durston's Custom Altair System

    Dual-drive custom FD360 Cabinet with Pertec FD400 Drives

    GW312 Serial to Ethernet Server (Supplemental Resource)

Tom Durston's System includes a pair of external Pertec FD400 drives that can read original Altair diskettes.

Tom Durston's Custom Altair Mainframe

(Not current configuration)
Tom Durston's Custom Altair System supports multiple configurations.

Dual-drive custom FD360 Cabinet
with Pertec FD400 Drives

Dual-drive custom FD360 Cabinet

Optional Board Page
Calculator Page
Supplemental Resource Page
Dual-drive custom FD360 Cabinet

Tom W. Durston's Custom Altair Mainframe

Tom W. Durston's 1975 mainframe with factory magic marker on the front panel.

Lighting can change darkness in a photo.

Note behind front panel dress plate.

MITS Engineering Machine born 4 Mar 75

(Bill Yates, MITS Head Engineer)

The back panel is pre-diskette controller, probably 8800a. The bracket in the photo is the temporary solution for the larger connector. Newer back panels have a cutout for the connector.

The back panel is pre-diskette controller. The bracket in the photo is the temporary solution.

Custom Front Panel Interface

Wires from the front panel connect to an S-100 connector. A short board connects to the front panel to the motherboard.

The board set pictured above does not match the current configuration.

Tom Durston's Custom Altair System supports multiple configurations.

Custom Front Panel Interface Connectors

Custom Front Panel connectors.

Older systems had the white wires connected directly to the motherboard. The 8800B has a special interface board. Tom Durston's mainframe has this one-of-a-kind removable interface.

Current Board Set Configuration

Left to Right, facing front panel:

  1. Empty slot
  2. Empty slot
  3. Empty slot
  4. Empty slot

  5. 88-2SIOJP with CTS on port 2 for
troubleshooting and
Original MITS 88-2SIO can be installed.

Both boards can be installed at the same time after addressing changes.

  6. No Connector, No Card Guides
  7. Custom Front Panel Interface
  8. No Connector, No Card Guides
  9. 8800B CPU BD
10. Empty Slot
Disk Controller 1/2.
12. Disk Controller 2/2

13. Empty Slot
14. 88-16MCS, 16K Static Memory
15. 88-16MCS, 16K Static Memory
16. 88-16MCS, 16K Static Memory
17. 88-16MCS, 16K Static Memory for a 64K System

88-PMC, 2K PROM Board for a 48K System plus 2K PROM

18. Empty Slot

"... many Altair hobbyists want to build up Rev 0 systems with all early MITS boards, the original low-wattage power supply, etc. These systems never existed in the real world! Tom Durston's system is a prime example -- upgraded bit by bit, improvements made along the way. "

Optional Configuration boards are available, but require additional test, repair, and configuration changes. Additional boards allow the system to support multiple configurations.

For example, removing the 88-2SIOJP requires adding the 88-PMC to get a boot PROM. Adding the 88-PMC requires removing the top 16K RAM. The new configuration does not allow the system to run MITS accounting, since 64K is required.

The example configuration above can support adding three 88-4K RAM boards. That still leaves a 2K gap and not enough RAM to run all MITS software. The 4K boards require a lot a power.

Tom Durston's Custom Altair System supports multiple configurations.

Tom Durston's 88-2SIO

BAUD Rate switch added!

MITS Test Department?

Port 2 Wiring (RS-232 with CTS):
  • E1 Black to J3
  • E2 Red to I2
  • E3 Yellow to J1
  • E4 Green to N8
  • E5 Green to N6

  • S2-1 Red to I1
  • S2-2 Black to J4 to Ground
  • S2-2, S2-1 N.C. (CTS Enabled)
  • S2-3 Green to N7
  • S2-4 Black to S2-10
  • S2-5 N.C.
  • S2-6 N.C.
  • S2-7 Yellow to J2
  • S2-8 Green to N5
  • S2-9 N.C.
  • S2-10 Black to S2-4 to Ground

MITS Technician Bruce W Fowler (BWF or FOWLER on boards).

Tom Durston's Original IO Cables

Tom Durston's original 88-2SIO cables included. New cables are provided with the system.

Port 1, RS-232 lines 2,3,7 (Console)

S1-4 (Black) to DB25 (Pin 7)
S1-7 (Red) to DB25 (Pin 2)

S1-8 (White) to DB25 (Pin 3)

MITS software does not require handshaking. The Console can be a three-wire connection.

Port 2, RS-232 lines 2,3,4,5,7 (CTS for RCPM)

S1-4 (Black) to DB25 (Pin 7)
S1-7 (Red) to DB25 (Pin 2)
S1-8 (White) to DB25 (Pin 3)
S1-1(Brown) to DB25 (Pin 4)
S1-3(Green) to DB25 (Pin 5)
"Proper" internal cables depend on the application and may include board modifications including a current limiting resistor on 5V.

The original MITS 88-2SIO board requires modification to enable CTS or other handshaking signals.

MITS 88-2SIO Board.pdf

Tom Durston's Console cable

Tom Durston's Console cable

Two modern cables are included with a 9-pin connector for a PC, USB to serial converter, or Ethernet Serial Server.

Tom Durston's 88-PMC (Included)

Optional 48K RAM Configuration
Optional 2K 1702A PROM with FDC Boot

14229 ENGR

Installing the 88-PMC requires removing the top memory board to make addressing space available to PROMS. The DBL PROM boots the MITS Disk Controllers.

PROM on the 88-2SIOJP makes the 88-PMC optional. The MITS upgrade would be a Turnkey Module (not included).

Mike Douglas has a great 1K monitor that is perfect for this board. The AltMon Altair Monitor adds function to a board that normayy only has a boot PROM.

AltMon Altair Monitor


Disk Controller Board #1
MITS INC 1976 8800 BD 1  REV 0-X4

Disk Controller Board #2

Tom Durston's Custom Mainframe was used to develop the MITS FDD controller and the FDD system!

CPU Connector

8800B CPU 

Note the resistor pack on the back of the 8800B CPU connector.

The star marks work of the MITS Technician Alan Haskins.

8800B Power Supply Board

The 8800B power supply board supports power hungry boards like the MITS 4K Static Memory board. The two original TIP-14x parts may fail someday due to heat or a short. The capacitors may fail someday too.

I hope the mainframe continues to be used, however, it may be wise to obtain appropriate repair components for later use.

Barely visable in the top corner is old repair for one connector to the 4-point bridge rectifier. A solder connection instead of a crimp.

Power Supply Filter Capacitors

Some Altair mainframes have a single large filter capacitor. This mainframe has two in parallel. Some people are afraid to turn on old hardware because old capacitors may leak, short, or otherwise fail. My experience, as of 2017, is that the big blue capacitors in Altairs still work.


One way to condition old capacitors is to use a Variac to bring up the line voltage slowly. Tom Durston sent me this design for another way to
condition old capacitors.

Here is the implementation of Tom Durston's design. The incandescent bulbs, 60W and 100W, are now vintage items. The idea works, but finding a 100W bulb will be hard over time. The incandescent bulb device was used on the mainframe (device not included).

Bottom view of the motherboard and the transformer tag

I found a tag that documents that the power transformer is a sample from the Heyboer Transformers company that made the 8800B transformer.

The 11/3/75 date indicates the mainframe power supply was upgraded
at least seven months after the mainframe was born.

The salesman sample transformer is bigger that the Attache transformer and smaller than an 8800B transformer.

TWD indicates the owner is Tom. W. Durston.

The orange switch is a fan on/off feature added by MITS Technician Alan Haskins.

Note the custom fan on/off switch and Velcro for the fan screen.

The tag provides some specifications for the transformer.

Tom Durston's 16K Static Memory Boards
(four boards, one board needs repair and a replacement is available)

Note: Black Plastic RAM package
Note: BWF, FOWLER on back

Note: S/N area and revision area after 200129

The images above show Tom Durston's 16K Static Memory boards.

Two 16K boards are Copyright 1976 and two are Copyright 1977, (part 200129). The mainframe was born
4 Mar 75 and 16K RAM was added in the next two years.

Four original 88-16MCS Static Memory boards passed the MITS MTST Memory Diagnostic and are used with an 88-2SIOJP to produce a 64K system

Three original 88-16MCS Static Memory boards can used with an 88-PMC to produce a 48K system with 2K of firmware.


Altair Mainframe Case Top with duct tape
to alter cooling air flow

These images record tape placement for an Altair mainframe. Duct tape is common in MITS single drive cabinets. The tape can be removed, however its use is part of Altair history.

Note: It is VERY IMPORTANT to install the top and screw it down tight, before shipping the mainframe. The top adds strength to the somewhat fragile outer case. The mass of the transformer can cause damage during shipment.

Boxer Fan Replacement

WS2107FL Boxer Fan (green label)

The original fan is damaged (included).
A Boxer Fan with a yellow label was used as a replacement.

A green label replacement Boxer Fan will be installed to match the original configuration.

WS2107FL 2R Boxer Fan (yellow label)

MITS Boxer fans have a green label. A green label replacement Boxer Fan will be installed to match the original configuration.

Back panel showing Velcro mounted fan screen, serial cables, and the fan on/off switch.

Continued on Dual-drive custom FD360 Cabinet
Continued on Optional Board Page
Continued on Calculator Page
Continued on 88-2SIOJP
Continued on Supplemental Resource Page