Reading paper tapes

Rhode Island Computer Museum sent out a request for PDP-9 paper tapes. As I happened to have some tapes it was just a matter of reading them and sending the tapes via mail. Easy. But I had no running computer that could do the job.

I decided to do a small interfacing exercise: Connect the old world of a PC04 reader/punch to a new world Arduino board.

This is a PC04 loaded with the valuable paper tape

PC04 front panel

This machine from the early seventies reads fan folded paper tapes at 300 characters a second. It's just a catch, it doesn't work stand alone. It need a computer to control it since it is completely stupid.

Arduino and DEC card

The Arduino board has to control the phases of the stepper motor. It has to receive the strobe pulses from the photo transistor and it has to send it to the host computer. The reader consists of an amplifier board G918 and four driver boards for the stepper motor.

PC04 inside

The capacitors are fairly big in this unit so I used a Variac to slowly increase the input AC to 115VAC which is the nominal voltage for this unit. The old linear power supply delivered voltages well within specification.

G918 board

I had to tune the photo amplifier (G918) using the trimmer to make it read all the bits correctly.

M040 board

Decent size transistors used for the stepper motor driver on the M040 board. All logic to control the stepper motor is on the M840 board (in a PDP8/a or PDP8/e) and consists of around forty mostly TTL IC.

More information regarding the PC04:

I put together this code to control the Arduino board:

This is the C code for the host computer:

After some debugging this was the output on the host. Looks like a PDP-9 tape in BIN format to me!

Source code could be found here.

A Youtube clip showing everything in action.


As the PC04 included both punch and reader and the mockup looked not so very good I made a PCB layout and ordered boards from China. This is how the board looks populated. 

Reader / Punch card

There are a number of boards left if someone need a board. But unfortunately they need some rework since there were some design flaws. The software and layouts are available here.