Sunday, May 27, 2018

Debugged and did more installation of the USB interface board into the Documation card reader.


A reader pointed me at right angle USB adapters, which have a good chance of allowing me to make use of my interface PCBs as constructed, allowing the cable to exit sideways and not interfere with the connector to the Documation electronics.

 Neither of my interface boards were communicating over USB after I had force fitted the USB cable to the mini (B) connector. I suspected that this was a result of damage to the fragile mini USB connector itself. I verified that by bending the inside of the connector, carefully, and restoring communication on one of the boards. The other was too damaged to resurrect.

I hope the right angle adapters will be safe enough to use with my boards, without stressing the mini USB connectors. When they arrive I will make a determination. The first set I ordered turned out to be regular USB A right angle adapters. The next set I ordered was a regular USB A to regular B right angle adapter. Only now have I gotten (I hope) the correct USB mini right angle to USB A cable.

An alternative would be to remove the connectors and somehow install bare wire from a cable to the board. Since the connector is surface mounted to the board, I would need a method of mechanically securing the cable end, thus some thought is still required for this possible approach.

I did some experimenting with the damaged board and found that it was infeasible to connect the wires directly to the surface mount pads on the board. It would take very fine wire to solder onto the five thin surface pads, then those need to be soldered to the normal stranded wires in a cable, all without a good way to provide strain relief.

My LED holders are here and I proceeded to drill and install the holder into the Documation 1000. The card cage cover can then be installed, leaving the rear cover off so that I can install the board and right angle cable when it arrives.

The rewired LED lights and toggles with the pushbutton, exactly as it should. The board communicates nicely with my PC, although I can't plug the connector with the card reader signals onto the board since my current cable protrudes over the connector space. Right angle connector will (should) be the solution.

EOF state toggled on
EOF state off

We located all the schematics and wiring diagrams for the unit we are restoring - the desk with its confusing myriad of sockets and wires, the transmitter unit and the printer/keyboard unit. We already had the wiring diagram for the REC 30 rectifier that converts the line power to 120V DC to power the current loops used by teletypes.

Teletypes encode text into 5 bit Baudot code, which can be shifted to Letters or Figures since 5 bits isn't enough to encode 26 letters, 10 digits and some other characters like period. One of the 32 codes is assigned to Letters, for example, to shift the machine to one state much as the Shift key on a typewriter shifts between lower case and upper case encodings of each key.

The 5 bit character is sent serially over the line, beginning with a start bit, the five data bits and a stop bit that is slightly longer than the others. Bits are sent by either the flow of a 60ma current (Mark) or no current (Space). The idle state of the line is Mark, so a steady current flows until a start character is transmitted by sending Space for the appropriate time.

The machines are set up to operate at a target bit rate - this machine has the most popular rate of 45.45 Baud (bits of a Baudot transmission) which is 22ms per bit time. Each character has 5 data bits, 1 start bit and 1.42 stop bit times, thus it takes just over 163 ms to send it. That is a sustained rate of a bit over 6 characters per second. Commonly, this is referred to as 60 word per minute speed, with an assumed average character length per word to make the math come out right.

Paper tape is used to store and send messages, in addition to the keyboard of a teletype machine. The paper tape has five holes, to represent the Baudot (actually ITA2) encoding as described above. Since each character on tape comes in with all five bits, but the data is transmitted serially over the line and serially to the printer itself, some form of parallel to serial conversion is needed.

The transmitter unit provides this function. It reads the paper tape and for each five bit character read, it spins a rotary encoding wheel on turn to send the start bit (always Space), the five data bits (Mark or Space depending on the associated hole in the tape), and the 1.42 bit long (31+ ms) Mark that is the stop bit. Thus, it is a parallel in, serial out shift register but done electromechanically.

Transmitter unit of the teletype (paper tape parallel to line serial converter)


  1. For your USB issue, would one of these help:

    A 180 degree usb adapter with down or up versions, assuming you can get one for the connector you have?


    1. This is for micro USB. The mini-USB is rarer now and has fallen out of favor, perhaps due to its fragility. Fortunately, I did find a right angle mini-USB cable that fits nicely.