MITS ALTAIR 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 the Greenkeys group recommended that we check the fuse in the reader power supply

Click on the image for a larger view

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
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
Without that cable connected the loopback program is now working with the Teletype

Loading BASIC didn't work, the Teletype reader stopped working after reading about half of the tape
That will be the project for next week.

The output of the power supply for the paper tape reader was 0V
The fuse in the power supply was blown again, but this time we found the cause

The 850 Ohm resistor in the power supply was cracked and allowed the faston connector to touch the case
We put an insulator between the faston and the case, and it is working OK now
We will have a replacement resistor by next Saturday

To load BASIC V3.2 we need to toggle in the loader found on the above link and shown below
The paper tape reader on the Teletype is used as N.8,2 because the 2nd stage loader is a binary tape

000: 076 003 323 020 076 021 323 020 (Change to 025 for 1 stop bit)
010: 041 256 017 061 032 000 333 020 (Change to 037 for 8k BASIC and 057 for Extended BASIC)
020: 017 320 333 021 275 310 055 167
030: 300 351 013 000

The first stage loader went into the weeds and didn't load the paper tape

We removed the two MITS 4k memory boards and the Cromemco Bytesaver and replaced them with a 64k Cromemco RAM board
Doing a deposit examine looks a little strange on the data LEDs

We removed the Cromemco 64k board and reinstalled the first MITS 4k board

The first stage loader looked like it was running OK while it loaded the send stage loader
After about half of the loader was loaded the first stage loader went into the weeds
Maybe a bad paper tape?

This loader loads the second stage loader from the paper tape
The second stage loader looks at the front panel switches to determine how to load BASIC
We have a 2SIO serial board, so switch A11 should be up
We are using a Teletype that is always configured for E,7,2 to load the second stage of the paper tape so switch A10 should be down

When the second stage bootstrap is running the front panel lights should display 07647

We have too many variables for debugging this system
We could have a bad CPU, RAM, Front Panel, Paper Tape, Backplane, or Power Supply

We will postpone further work on this system until we can get some help debugging the the individual parts
The Boston Vintage Computing Meetup will be at the RICM on March 7
We asked their Altair experts to bring a running system so we can swap parts and determine what parts in our Altair work

We received the replacement resistor for the Teletype and replaced the cracked one
We finished the assembly of the Teletype, and declared it complete

The Boston Vintage Computer Meetup group visited our museum today and brought a working Altair 8800
Now we know the Teletype reader is working OK

We tried the first Altair 4k Dynamic Memory board in the  
It did really strange things when   tried to run Killbits
That confirmed the strange behavior when we tried to load BASIC on our Altair

We jumpered the second Altair 4k Dynamic Memory board to start at address zero and tried it in  
Killbits worked OK, so this memory board at least works better
BASIC V1 didn't load correctly, so maybe the second memory board is bad too

We have been told by many experts that we should not even try to get these 4k memory boards to work
We will look in the warehouse for another memory board

Another visitor brought an EPROM programmer and read the 2708 EPROMS that were in the Cromemco Bytesaver
We will disassemble the EEPROM contents to see what we have

To do:
  • Document the jumper settings on all boards (took pictures)
  • Determine how the serial port is configured (110 baud, 20 mA Current-Loop)
  • Implement the second serial port (need to buy a UART)
  • Disassemble the software in the EPROMS in the Cromemco board
  • Repair the front panel (done)
  • Get the ASR33 Teletype working and connected (picked up 12/31/19, everything OK)
  • Punch Micro-Soft BASIC on paper tape (done)
  • Try loading Micro-Soft BASIC from paper tape on the Teletype (Not successful so far)
  • Program Micro-Soft BASIC and a loader into the EPROMs on the Cromemco board