PDP-8 and PIP

----- Original Message ----- From: "Marco Rauhut" <marco at familie-rauhut.eu> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> Sent: Friday, August 15, 2014 5:37 PM Subject: PDP-8 and PIP Hi, there was a post on alt.sys.pdp8 years ago about modifying the M8655 to output the reader run signel as RS232 to give hardware flow control, I dont have the origonal link but a copy of the post is below, not tried it yet but its on my "must try soon soon" list Dave H --------------------------------------------- aron Nabil More options Dec 7 1998, 8:00 am Why would you want to do this? To get "reader-run" support. What is "reader-run" support? If you have an ASR-33, it's the thing you probably don't have (unless the ASR-33 was original DEC equipment) that allows the -8 to control the paper tape reader. It's an extra PC board that fits inside the control housing that requires another pair of leads from the -8 (so now there are 6 total) that signals the teletype when to send data. What's it used for? You can turn the reader on, and the -8 will start and stop it. This is required for multi-part paper tapes, since the -8 will stop the reader after the first section, and you need to press "continue" to get it to read the rest. Without reader-run support, the reader will just keep playing into the next segment (the computer won't see it, it's halted!). Having long ago given up on Teletypes, I want my KL8 to work on my PC. Same problem, no control of the "reader" (in this case HyperTerminal), so tapes just keep playing even though the computer isn't listening. Solution? Hack the M8655 to support reader-run by mapping it to RS-232 hardware flow control. Here's how you do it. 1. Cut the trace leading from Pin 1 of E54 (a 7400). 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 3 of E45, a 1489 RS-232 receiver. This supplies the signal to the reader-run flip-flop 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 EIA receiver (thru a needed inversion). 4. Cut the ground traces that run to pins 4 and 5 of E50, a 1488 RS-232 transmitter. This is what would normally supply the continuously asserted RTS signal. 5. Jumper from pin 7 of E39, a 7474 flip-flop to pins 4 and 5 of E50. E39 is the "reader-run flip-flop". This works great, except for a couple minor glitches. You've just destroyed the current-loop reader-run capability of the board. I'll leave it as an exercise for the reader how to modify the board to support both at once with the limitation of the number spare gates present on the board. Hyperterminal, when set to RTS/CTS flow control, will silently hold the rest of the paper-tape (garbage, probably) until RTS goes high again (by this time in the application program) and send it at the most inopportune time, like to TECO which you just loaded. I get around this by closing hyperterminal and opening a new one. I've only loaded one interactive program so far, FOCAL '71. For some reason it didn't set the reader-run on the first character (but did on all the subsequent ones). I've gotten around this by switching to a Hyperterminal session with flow-control disabled. Since I closing the existing session to get rid of the trailing garbage still in the buffer, this isn't particularly inconvenient. You need to make sure your paper-tape images start with a valid leader only. I used a binary editor to "fix up" the paper-tape images I got from Cam (there are copies of these 164 tapes at)... http://www.spiritone.com/~nabil/pdp8/camtapes Caveats: Get Hyperterminal "Private Edition 4.0" (free). Earlier version had problems with NULLs. It lives at... ftp://ftp.hilgraeve.com/htpe/htpe4.exe For running interactively with the -8, set hyperterminal to coerce all input to 7 bits. I was having problems until I removed the extended status jumper on my M8655. I don't know if that was really the problem, but it started working when I removed it. I set both the board and HyperTerminal to 8/N/1. A truly clever hacker would make a RS-232 shell with a push-button in it the forces RTS, thus both fixing the problem of having to dump the garbage data and the no RTS on first character. -- Aaron Nabil na... at teleport.com