Tuesday, October 22, 2019

Evaluating restoration of integrated control unit from Telex 8020 cabinet A


The integrated control unit in this Telex 8020 tape subsystem is a plug compatible alternative to the IBM 3803 which requires its own cabinet. The control unit front end attaches to an IBM mainframe using a pair of hefty parallel cables called Bus and Tag. This control unit causes the mainframe to believe there is a string of IBM 3420 tape drives available; it responds to read, write, control and sense commands just as the 3803 would.

This control unit back end attaches to each of the tape drives in the string, using the simple Telex protocol with the drives. The parallel cable used for the connection has lines for the 9 data bits read from the drive, 9 data bits to be written to the drive, plus status and control lines. It can command these actions and more from the drives:
  • Read a record
  • Write a record
  • Read one record backwards
  • Space forward one record
  • Write a tape mark
  • Rewind
  • Rewind and Unload

I will be testing each drive individually with the standalone Telex Tape Tester, a device which plugs into the drive and controls the signal lines that normally would come from the tape control unit. Thus I will know if the drive is correct as far as movement commands before I get the control unit working.

What I don't have is a mainframe or channel tester that I could use to drive the control unit to verify that the front end works properly. What I would need to do is send various channel commands to the controller and read both data and the sense (status) data from the control unit. For this to work, the control unit must properly respond to the signal protocol on the tag cable and transfer the correct data or sense bits on the bus cable.

I may be able to produce test sequences of pulses using my Analog Discovery and capture the resulting pulses from the control unit on my logic analyzer, for simple sequences at least. This could partially test the control unit.


The wiring diagrams show that the control unit is fed 240V from two legs of the 3 phase master input, just like the power supplies in cabinets A and B. However, I couldn't find any large socket on the control unit for power. It appeared to be completely featureless on the outside, only having ribbon cables coming out to the bus/tag connectors and up to the two tape drive card cages.

I finally spotted a tiny US style male socket recessed up on the rear of the control unit,which must be the power input. Totally unlike the connectors for the supplies, so it must draw far, far less current in order to use the widespread 15A connectors.

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