There are numerous tales on the web of restoration of Altair 8800 microcomputers. This is another such tale, but with some difference. This MITS Altair 8800 was received for restoration (along with an 8800b) but was in quite poor condition. Notably, two of the three power supply transformers, including the larger +8 one, were missing.

The Altair as originally released had various shortcomings and awkward aspects to its design. MITS actually produced three versions of the Altair, rectifying many of the shortcomings:

• Altair 8800
• Altair 8800a: Heftier power supply, more bus slots.
• Altair 8800b: New look to the front panel, additional front-panel alterations, better front-panel logic design.

Given the poor condition of this unit, difficulty of restoring it to originality, and the original shortcomings, the decision was made to instead implement improvements on the design in the course of restoring it to functionality.

The result presented here will be referred to as the 8800i - "i" for improved. The improvements have been made with an eye to avoiding anachronisms - that is, these improvements could have been performed in the mid/late-1970s period of the Altair by a motivated owner, in the manner presented here. The subheading above then, is somewhat tongue-in-cheek, in that this article too could have been written in the 1970s, but isn't intended to suggest taking an 8800 today in good original condition and making all these modifications.

Contents (this page): Altair 8800i Improvements Photos The Popular Electronics January 1975 Article

Sub-pages: Unit Log Unit Configuration Altair 8800i Schematic (pdf) Altair 8800 Schematic (pdf) MITS 88-CPU 8080 CPU Schematic (pdf) MITS 88-2SIO Dual Port Serial I/O Schematic (pdf) MITS 88-4PIO Quad Parallel I/O Schematic (pdf) PROM Programs, including 3 blinky programs

Related Pages: Altair 8800b S100 Miscellaneous References / External Links: Altair 8800 4-slot bus-extension reproductions from Trailing Edge Technology. AltMon at altairclone.com, monitor derived from Vector Graphic monitor. Altair BASIC interpreter and programs at altairclone.com. More software at altairclone.com. Altair at deramp.com. S100 Computers, reference site with extensive S100 board documentation.

Altair 8800i Improvements

• Power Supply.
  • The power supply current abilities are increased.
  • The two +5 sources are integrated into one.
  • The three step-down transformers are replaced by two. The rectifier and filters circuits are consequently also simplified.
  • The power supply location is moved to the right-side of the cabinet for cleaner organisation and access. The power supply is a removable module rather then being integrated into the cabinet.
  • There is space and provision for a small third transformer to be added to act as an auto-transformer on the line/primary side. This can be used to buck the line voltage down in circumstances of higher line voltage and/or light loading on the S100 bus to reduce power dissipation in the on-board regulators. This bucking can be applied to either or both of the step-down transformers as suits operating conditions. The original +5L,+16 transformer can be used as the auto-transformer.

• Extended S100 Bus. With the power supply occupying the right-most quarter of the cabinet there remains room for 12 S100 bus slots.

• Front Panel Wiring. The 8800 stock design had the front panel hardwired to the bus with a large bundle of fine wires, with no strain relief provision. This makes servicing the front panel awkward. In the 8800i, this bundle of fixed-wiring is replaced with connectors and ribbon cables to an interface board that plugs-in to the S100 bus. While there are various connector combinations that could be used in accomplishing this, for this unit, edge pins were cut down from surplus boards and mounted to the front panel PCB, then the edge pins wired to the various points on the PCB. While this is a bit of a kludge, it nonetheless results in the front panel being a removeable, serviceable, and solid module. See photos.

  The new front panel interface board was constructed on an NOS S100 prototyping board. In hindsight, given the price of S100 protyping boards these days, it may have been more cost-effective - and certainly more labor-effective - to have done the layout design for manufactured PCBs.

• Front-panel Data LEDS Output Port. The front-panel Data LEDs can be used as a display during program execution by writing to I/O Port 0xFF. This is a feature that was implemented in the 8800b. It can be provided by the addition of 4 ICs on the mentioned front-panel interface board.

• Front-panel RDY Signal. The 8800 uses pRDY (S100.72) to start/stop the CPU for front panel operations. pRDY is a shared assert-LOW signal open for use by other boards. The 8800 front panel however, rather poorly, uses a totem-pole driver for pRDY rather than an OC driver. The 8800-i provides an OC driver for this signal. As well, a jumper is provided to choose between driving pRDY (S100.72) or xRDY (S100.3).

• IEC Power Connector. The fixed line cord of the original is replaced with a standard IEC power connector.

• Primary Fuse. Moved to fuseholder on rear panel.

• Front-panel Power switch. In the original design, the power switch and its connections were soldered to traces on the front-panel PCB. These live-mains connections are very close to logic circuitry, and the fixed connections add to the awkwardness of removing and servicing the front panel. Here, the PCB is notched to free the power switch from the front-panel PCB. The wire connections to the switch can then be insulated with some form of plastic goop such as hot-melt glue.

• Fan. With switch on rear panel so it can be turned off when/if not needed.

Photos

Front panel. This unit apparently did see some use in its day as evidenced by the lettering around switches worn-off by fingers.	Rear panel. Modified with IEC connector replacing the original fixed line cord, and fuse holder.	Refurbished and functional.
Another internal view.	New power supply module.	Closeup of the interconnect between the front panel and new front panel interface board.
Installed power supply module.	Power switch removed from the front-panel PCB, and insulated.
Front panel board.	Front panel board, solder side, with new edge connectors wired in.	New front panel interface constructed on an S100 prototyping board.
Original MITS Altair CPU board. The board employs an 8080 microprocessor running at 2MHz.	MITS 88-2SIO dual-port async serial I/O board. Two Motorola 6850 UARTs are used. Configurable for RS-232, 20mA current loop, or TTL interface; and various bit rates from 110 to 9600 b/s.	MITS 88-4PIO parallel I/O board. With four Motorola 6820 PIO ICs this board will actually provide eight 8-bit parallel IO ports, along with 2 control lines per port.
Simple ROM board providing 2KB in a 2716, constructed on an S100 protoyping board.	The chassis - more-or-less as received - prior to reconstruction.

The Popular Electronics January 1975 Article

The cover of the January 1975 issue of Popular Electronics.

First page of the Altair article. Note the case and front panel are actually quite different than the 'actual' Altair 8800.

The guts of the Altair as presented in a photo in the article. This assembly bears little resemblance to the Altair 8800 as actually shipped.