Friday, July 27, 2018

Modification to allow use of ASR 33 paper tape reader with PDP 8 clone (SBC 6120)


I was exchanging emails with a DEC computer collector and mentioned that the SBC 6120 didn't have the signal output to switch the Reader Run Control relay board in the ASR 33. Without the signal, the motor of the punch tape reader runs at full 110 baud speed which is too fast for programs in a DEC minicomputer to keep up.

To avoid overruns, the serial board in the PDP 8 will switch on and off a signal driven by software. The software is interrupted when the serial board has received a character into its buffer and can interrogate a flag indicating the data is ready for transfer. A program will fetch the character and then issue a command to reset the flag.

When the flag is on, the Reader Run Control relay is switched off pausing the paper tape reader motor. After the software clears the flag, the relay turns back on and the paper tape reader advances to the next character.

I looked over the schematic of the SBC 6120 and examined the data sheet of the Harris 6402 UART chip it uses for serial data. The clone uses the Data Ready pin from the 6402 as the flag that is visible to software, and the command to clear the flag toggles the Data Ready Reset pin.

Pins 18 and 19 of the Harris 6402 chip

Schematic portion with 6402 UART chip wiring
Thus, I can tack a wire on the 6402 to watch the Data Ready signal, invert its state and convert to an RS 232 level signal to drive the relay coil of the Reader Run Control board inside the teletype. I will have to scramble to build the circuitry and do the modifications, then test this to see if I can successfully read in programs from paper tape when running on the SBC 6120. 

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