Contents
- General
- My Altair 8800 in Disguise
- S-100 Boards
- Altair Clones
- Altair Duino Pro by Adwater & Stir
- Altair 8800 Clone by Mike Douglas (extern)
- Simulators
- Altair 8800 simulator by Peter Schorn (extern)
- Information
- Links
General
The Beginning
If you want to delve deeper into the MITS-Altair, you will find endless information and software on DeRamp.com. What you don't find there probably won't be found on the rest of the internet!
The Altair Font
I do not (yet) know what name this font had in the 70s. Today it is 256 Bytes and is from TYPODERMIC FONTS INC.
The logotype for the MITS Altair 8800, the first commercially successful personal computer, inspired 256 Bytes. Unusual thick-thin choices are reminiscent of 1960s MICR types like Data 70 and Westminster. [2]
My Altair 8800 in Disguise
Since I now have three S-100 systems (Northstar Horizon, IMSAI 8080, SOL-20) I don't want to buy another one. So I thought by myself, why don't you take one of the three IMSAI's and just make an Altair 8800 out of it. No sooner said than done and here is my first attempt with a 5.25" Altair Mini Disk.
My Altair 8800 (yet) consists of:
- IMSAI MPU-A CPU card (I will replace it with a brand new Altair replica (MITS CPU BD REV-1)
- Wameco FPB-1 front panel
- Gould ICC 100-0062 serial I/O card (I will replace it with the 88-2SIOJP by Martin Eberhard)
- Altair FDC+ (FDC, RAM, ROM) by Mike Douglas
I established the connection between the 50 pin FDC+ connector and the 35 pin floppy drive with my universal terminal block breakout connectors; works fine!
Altair 5.25" Mini Disk
... to be continued ...
Altair 8" Disk
... to be continued ...
S-100 Boards
8800 1K STATIC MEM BOARD
One problem with this boards are the "sockets" for the RAM ICs. These consist of individual tiny metal pins. These must not touch each other under any circumstances!
Yes, these pins are really a bit whimsical. By chance and by simply swapping RAMs, board #0205K now works perfectly. All the RAM ICs are ok.
The boards are different. Pin 7 (G, 7432, red dot) of the upper board is not conntected to GND, therefore the additional line on the back.
I tested all ICs, all are working fine.
Special features
#0205K
This board really seems to be one of the first (assembled in 1975), especially as it was probably only used in the original version. [1] The two silver capacitors (CDE, date code 7331, NLW, 35-50, 35 MFD, 50 WV, 105 degrees), which were apparently replaced by blue capacitors in the later versions, are striking.
Unfortunately, all TTL ICs were soldered directly onto the board. The date codes are: 7327, 7328, 7332, 7402, 7406, 7414 and 7507. The oldest ICs (3 x SN74L00N) are thus 50 years old and working in 2023, see below!
RAM ICs
This board uses 8 x 1K (256 x 4bit) static RAMs: AMD 9101ADC, C2101-1, 7526P. Access time: 500 ns.
1K Block Addressing
See the picture below for the addressing scheme!
Testing
I tested the board in my North Star Horizon. There is only one important point to note. Altair and IMSAI use a slightly different S-100 bus. This affects signals 20 and 70. Since both are exactly opposite each other, it is very easy to glue them on with a very narrow adhesive strip.
For this memory test I have selected the start address F000. The free area (F000-FFFF, 4K) is wonderfully suitable for memory tests on the Horizon. It just must not be more than 4K!
- A15 = 1
- A12 = 1
- A14 = 1
- A10 = 0
- A11 = 0
- A13 = 1
- ====> 0^10 + 0^11 + 2^12 + 2^13 + 2^14 + 2^15 = 61,440d
- ====> F000h
I have completely renewed the address selection, as it was not usable as it was. It was also terribly "soldered" by the previous owner. Now it looks perfect and serves its purpose.
After this small tape correction, I was able to test the board extensively with the Rasmussen MEMR test. Result: The old RAM ICs (AM9101 and Intel P8101 from 1975) and the TTLs are in perfect order. Also the capacitors do not cause any problems. The voltage at the 7805 is exactly +5V.
Update 07/16/2023: I cannot comprehend it! Sometimes the board works perfectly, but a short time later it doesn't work at all. I have not changed anything. Sometimes the RAM ICs are OK and other times the RAM test only generates errors. I cannot comprehend it! Sometimes the board works perfectly, but a short time later it doesn't work at all. I have not changed anything. Sometimes the RAM ICs are OK and other times the RAM test only generates errors. It is maddening!
Update 02/08/2024: I have now tested the #0205 board extensively (MEMR) on my IMSAI. Until now I always had to jump to the memory address with the front panel and switch the board to UNPROTECT, because it always started in PROTECT mode. I then ran the memory test for 30 minutes; everything worked.
#2422K
RAM ICs
This board uses 8 x 1K (256 x 4bit) static RAMs: Intel P8101, S1010, R0527, 7514 Philippines. Access time: 850 ns.
1K Block Addressing
- A15 = 0
- A12 = 0
- A14 = 0
- A10 = 0
- A11 = 0
- A13 = 0
- ====> 0^10 + 0^11 + 0^12 + 0^13 + 0^14 + 0^15 = 0
- ====> 0000h
Testing
All TTLs, all RAMs, all capacitors and the +5V line are OK but it won't work. I have tested almost all the wires (resistance measurement) to see if the vias are ok. I can't find the fault.
Information
External Links
- Mike Douglas: DeRamp.com - Altair/MITS
- Peter Schorn: Altair 8800 simulator
- Altair Font: 256 BYTES
References
- (↑) Popular Electronics, Jan 1973, page 38
- (↑) Altair font: 256 BYTES, https://www.dafont.com/256-bytes.font