PDP-8/e

[Bob Rosenbloom]

On 12/7/2018 7:01 PM, Pete Turnbull via cctalk wrote:
> On 07/12/2018 17:44, Jon Elson via cctalk wrote:
>> On 12/07/2018 11:22 AM, systems_glitch via cctalk wrote:
>>> Indeed, unless you need character pacing.
>>>
>>>
>> Actually, with the correct settings of the serial port (xon/xoff or 
>> CTS pin) the serial port driver should do this, too, so cat would work.
>
> A PDP-8/E doesn't have a CTS pin and the loaders don't support 
> XON/XOFF, though.
>
The PDP-8 needs to control the serial CTS function. This was called 
reader-run when using a Teletype machine. FOCAL won't load without it.
You can modify the serial card (mine was an M8655) to support the 
function. Here's what I did:

Cleaned up from Aaron Nabil's and Lyle Bickley's write up.

  Hack the M8655 to support reader-run by mapping it to RS-232 hardware 
flow control.

1. Cut the trace leading from Pin 1 of E54 (a 7400).  This is the input 
that clears the Reader Run FF when a new character starts to come in.

2. Jumper from Pin 1/E54 to Pin 3/E38, a spare gate on a 7400 that we 
are going to use an inverter.

3. Tie Pin 1 and Pin 2 of E38 together, and run them to Pin 20 of E19, 
the UART.
     This supplies the signal to the reader-run FF that tells it that 
it's got an incoming character and to de-assert the reader-run line.
     Normally this is tied to the current loop receiver, we've just 
moved it to the UART so any received data will clear the FF.

4. Cut a ground traces on 4 of E50, a 1488 RS-232 transmitter. This is 
what would normally supply the continuously asserted RTS (and DTR) signal.

5. Jumper from pin 7 of E39, a 7474 flip-flop to pins 4 of E50. E39 is 
the "reader-run flip-flop".  Now RTS follows the reader run signal.

Bob

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