MITS ALTAIR #1 Restoration


The front panel is misbehaving, but we can get the processor to run and blink the lights on the front panel. Pressing EXAMINE does not load the switches into the address, and increments the address. Pressing RESET sets the address to 0000. Pressing EXAMINE NEXT increments the address and displays some data

The D3 LED is always on
We checked the SN74lS04 that drives the LED, and we see a 30uS pulse when RESET is pressed, so that means the LED driver chip is OK. we pulled boards from the system hoping that we would find one that was hanging the D3 bus. With all of the boards out we individually grounded DI0 through DI7 pins and each LED went out. After fiddling with the boards the D3 LED decided to behave normally


We removed all of the boards to take pictures of both sides and document the jumper settings. After reassembly the front panel looks like it is working OK now.


We picked up the ASR-33 Teletype that was used with the Altair. The printing unit was not mounted to the base and the H-bar that connects the keyboard to the printing unit is missing. It looks like this Teletype has both current-loop and RS-232 interfaces. It is going to take some work to get this Teletype working again


We analyzed (fiddled with) the ASR-33 Teletype today. If you manually hold the part in the printer assembly that connects to the H-bar, clutch disengages correctly. Pressing a key on the keyboard and manually allowing the clutch to engage prints a character. The same is true for the paper tape reader. Next week we will visit the warehouse and get the missing parts, and hopefully get this working correctly. We will also swap the connector on the serial cable and the Teletype to match what we use on the DEC computers .


We raided the warehouse for the missing ASR-33 Teletype parts. After some cleaning and reassembly it seems to be working nicely.

We even connected it to a Windows laptop through the RS-232 interface and printed a test page. We also found a spare Cromemco 64k memory board that we can use in the Altair if the MITS 4k memory boards misbehave.


Alex cleaned the top cover of the Teletype, and it looks very nice. We repaired a crack in the cover with epoxy, looks better

We punched a paper tape of Altair BASIC V3.2 on the Teletype. Time to see how the remote reader control works.


It looks like the additional reader control circuit is only controlled by the 20 mA current loop interface, and not the RS-232 interface. There is a small switch added to the reader control circuit to enable the reader when the RS-232 interface is used. The distributor clutch is constantly being tripped when there is paper tape in the reader. The magnet is the reader is not being activated, so the paper tape is not being read or advanced. There is a switch on the distributor that opens when the shaft is released to advance the paper tape and engage the reading pins. We need to check the whole circuit from the power supply in the base, through the reader control board, and the distributor switch to insure that everything is working.


Back to working on the Teletype. Too many visitors today to get much accomplished


This shows the paper tape Reader Control board that allows an external computer to control the reader with a 20mA control signal. Our DEC computers and the Altair 8800 have the 20mA control signal for the paper tape reader. Since this Teletype has an RS-232 adapter, and the RS-232 interface doesn't have the control signal for the paper tape reader, a bypass switch was added.


Wayne from Greenkeys suggested we check the fuse in the paper tape reader power supply. It was blown, so we replaced it. The reader still didn't work. We disconnected the wire harness from the reader power supply. The reader feed contact and the reader feed magnet tested OK. Of course, after we reconnected the wire harness the paper tape reader works! Next up, get the 20mA interface working. The RS-232 adapter is wired to plug #2 which is in parallel with the barrier strip. Just unplugging the RS-232 adapter will let us use the 20mA interface. We compared the the Molex connector wiring to the Teletype that came with the PDP-12. The transmit and receive wires were swapped, so we fixed that. We tried the Teletype as the console on the PDP-9, and it didn't work. More debugging to do.


The paper tape Reader Control board has a diode across the relay coil to stop the back EMF from blowing the transistor in the Altair. This means that the the polarity of the signal controlling the relay is important. Plugging the Teletype into the PDP-9 console cable caused a 3A fuse in the I/O controller to blow. It is likely that I have the wires for the Reader Control relay backwards so the diode is acting like a short. I will check that next Saturday. This is how the Reader Control board is wired. We need to make sure that terminal-3 is more positive than terminal-1, or the diode will conduct and look like short.

Teletype Molex Connector Pinout According to DEC

Pin Color Description TTY Altair 88-2SIO
1 N/C
2 GRN Keboard- T-3 -> 6 Receive+ Ground to -470 Ohms
3 WHT Printer- T-6 <- 4 Ground
4 BLU ReaderRun- RR-1 <- 10 Ground
5 RED Keyboard+ T-4 -> 7 Receive- -15V through 220 Ohms
6 ORN ReaderRun+ RR-3 <- 3 RTS+
7 BLK Printer+ T-7 <- 5 Transmit+
8 N/C

Altair 88-2SIO Jumpers for Teletype

Receive D3(REC)=K12 K11=Y7 Y8=S1-6 Y9=S1-7
Transmit D5(X-Mit)=L1 Z2(+5V through 220 Ohms)=S1-5
Ground S1-4=S1-10(Ground)
RTS D4(RTS)=L2 Z4(+5V through 220 Ohms)=S1-3

Once we have verified that everything on the Teleetype works OK we will make a matching console cable for the Altair. Then we can try loading Micro-Soft BASIC.


More work on the ASR-33 Teletype. The orange and blue wires on pins 1 & 3 on the reader control board were swapped compared to the DEC standard. That would explain why this TTY blew a fuse when connected to the PDP-9. We swapped the wires to match what DEC does, and to match the console TTY for the PDP-12. The TTY works OK as a system console on the PDP-9. The paper tape reader only reads one character and then stops. The problem is in the PDP-9, not in the Teletype.


We made a new console cable for the Altair with our standard 10 pin Molex connector on the end. That will let use use any of the three Teletypes, the VT220 terminal, or using a converter a PC as the console. The current loop interface keeps the TTY quiet when the Altair is turned on, so that is progress. A toggled in echo program doesn't work with the TTY, so we can't load BASIC yet.


The toggled in echo program that we found was set to use ports 00-03. Our 2SIO that is set to use ports 20-23, the standard for BASIC, so it did not work. We will go through the 2SIO board debugging described here. This echo program should work with our 2SIO board.

000: 076 003 323 020 076 021 323 020
010: 333 020 017 322 010 000 333 021
020: 323 021 303 010 000

This echo program didn't work either, time to get the 'scope out to see if the current loop interface is working. The signal from the Teletype on the 2SIO board look very noisy, so I can understand why it is not working. The Teletype works OK in local mode, so the distributor is working OK. Maybe just a bad connection?


Hugh, a new visitor with Teletype experience, pointed out that the cable to the RS-232 adapter had a jumper wire in it. It was probably connecting the keyboard and printer 20mA circuits and causing problems. Witho