The wild success of the MITS Altair 8800 in 1975 kicked off the hobbyist-computing craze in the mid-70s and inspired numerous competitors to enter this new and burgeoning market. The 8800 had significant shortcomings however, and MITS risked losing marketshare - especially to the IMSAI 8080. MITS had to 'up their game' to remain competitive.

An interim model - the 8800a - was released with a beefed-up power supply and more bus slots. This was followed in 1977 by the 8800b with a more comprehensive set of improvements:

• Full-coverage plastic translucent faceplate over the front-panel LEDs. This was far more durable than the silk-screen lettering on the 8080 which could wear off from finger-rubbing around the switches.
• Extended-length flatted-toggle switches on the front-panel. Far easier to flip than the common mini-toggles of the 8800.
• 4 new front-panel operations: Load & Display Accumulator, Port Input & Output with Accumulator.
• Drive buffers between the bus and front-panel LEDs. The 8800 drove the Address and most Status LEDs straight off the bus.
• Plug-in interconnect between the front-panel and bus.
• Synchronous-design front-panel.
• 18-slot bus on a single heavy-duty PCB.
• Beefed-up power supply with +/-18 pre-regulators.
• Chassis fan.
• 8224 clock generator on the CPU board, rather than the monostables of the 8800 CPU clock.

The 8800b was still based on a 2MHz 8080 CPU.

By this time, manufacturers were playing somewhat fast and loose with the non-standardised S100 bus. With the 8800b, MITS furthered the confusion by using several more bus lines for CPU-to-front-panel interaction, different again from the 8800. With this, the 8800b front-panel operation is tied to the unique 8800b CPU board.

Contents (this page): Restoration CPU Board Alterations Photos

Sub-pages: Unit Log Unit Configuration Altair 8800b Schematic Altair 8800b (Reconstructed FP Interface) Schematic Altair 8800b Front-panel ROM

Related Pages: Altair 8800i S100 Miscellaneous References / External Links: MITS88-CPU board reproductions from Trailing Edge Technology. JAIR board manual at s100computers.com. See V2.4 download. JAIR notes at retrobrewcomputers.org S100 Computers, reference site with extensive S100 board documentation.

Restoration

This unit was received for restoration. It had long been disused, a little grubby, but in pretty fair condition without much irreversible alteration or damage. 8 holes had been drilled in the sides, presumably for rack-rail mounting. One switch toggle was broken, a couple of others bent, one of which also later broke. Longer toggles do have the disadvantage of being more prone to being caught and more force leveraged at the throat. The toggles themselve are plated bronze, and a hollow thin-wall pipe, not a solid rod, so really not very strong.

There were no memory boards, but most significantly, both the special CPU board and front-panel interface board were missing. A guess is at some point the unit had been 'upgraded' to a 4MHz Z80 CPU board. This would have left the front-panel non-functional and the interface board unnecessary, the original CPU and front-panel interface boards then being lost or discarded.

To get the front-panel operational, the interface would have to be recreated. This was done in a one-off and laborious effort using an S100 prototype board. A few alterations were made in the course of construction to avoid use of the 8800b unique interface-to-CPU lines on the S100 bus so the original 8800 CPU board or other 8800-compatible CPU boards could be used.

In restoration, the unit was thoroughly dismantled for cleaning, and the power supply rewired. The original electrolytic caps have been fine, as expected for 70s-era caps.

The front-panel uses a 1702 EPROM to control a small sequencing engine for execution of front-panel operations. This EPROM turned out to be faulty. Rather than obtaining and programming another 1702, an adapter for a 2716 was constructed - the 2716 being far more prevalent and easier to program. The data to be programmed into the EPROM is fortunately available in the 8800b manual.

A JAIR board [ref] obtained earlier has been installed. This is a modern board with numerous configurable features. For purposes here, it is being used to provide console SIO and SD-Card-based storage in substitute for disks. The SD-Card storage allows one to boot to CP/M in some tens-of-seconds. See Unit Configuration.

CPU Board Alterations

• The clock oscillator has been altered to use feedback bias on the inverters rather than fixed bias.
• The timing resistor for Φ1 has been reduced to 10K rather than 13K, shortening Φ1 so as to gain a little more time for the Φ2 to Φ1 delay which was close to the 8080 spec minimum.
• Three 750Ω resistors are used for each of the 8080 clock drivers, rather than two 470Ω.
• The dropping resistor for the -5V regulator circuit has been increased to 820Ω/0.5W from 220/2W, reducing the regulator power consumption. The -5V is used only for the 8080 gate bias (Vgg), around 1mA. The zener used is a 1N751A and the electrolytic filter capacitor has also been reduced to 3.3uF.
• The dropping resistor for the +12V regulator circuit has been increased to 51Ω from 33Ω for the higher +18V supply of the 8800b compared to +14::16V of the 8800.
• 74LS367 bus drivers are used instead of 8T97's. While the LS367 have a lower drive capability than the 8T97, this should be tolerable in the 8800b as it has buffers for the address and status LEDs, in contrast to the 8800 which drives those LEDS right off the S100 bus.
• IC L is a 74LS74 rather than 74L74. The L74 may actually not be fast enough to provide adequate HOLD/READY setup time for the 8080, depending on the length of Φ2.

Photos

	The IEC power connector is a modification, the stock model has a fixed line cord.	Internal view.
	Closeup of connection between front-panel and replacement front-panel interface board.	Rewired power supply.
Front-panel hardware. The front-panel requires an additional board to interface and connect to the bus.	Replacement front-panel interface board.	CPU board, with 8080A running at 2MHz. This is a copy of the original Altair 8800 CPU board (not 8800b), using the PCB recreated by Gary Kaufman (ref). A few deviations from the original have been incorporated here. See text.
A 2716 EPROM adapted to replace the failed 1702 front-panel ROM.	As received, prior to reconstruction.