On Thu, Aug 18, 2016 at 09:08:59AM -0700, Vincent Slyngstad wrote: > From: Pontus Pihlgren: Wednesday, August 17, 2016 11:26 PM > >I've been running and looking at the PDP-8/E INSTRUCTION TEST 1. And it is > >supposed to write the BEL (07) character after each round of passed tests. > >However the code loads and outputs the constant "0207", which is 07 with > >the eigth bit set. I think the 8/E serial interface outputs all eight bits > >in this case? > > Yes; the serial interfaces were also used to send and receive 8-bit binary > as well as ASCII, so 8 bits are sent and received.
This is what had me confused, the ASR TTY is 7-bit but the ASR punch is 8-bit. > > >I suppose a real ASR-33 would ignore the eight bit? > > > >Why does the code not load 07? > > The teletypes used were typically set for "mark parity", so the ASCII input > data had the high bit set. The program is presumably mimicking the code > received from the TTY if BEL were input. The TTY will indeed set the bit > for keyboard input (paper tape in/out is 8 bits), and ignore it for > teleprinter output. Aha! I, in my somewhat more modern mindset, imagined that the serial interface would handle parity but I realize now it probably makes more sense to do it in software and keep the hardware simple. Thank you for clarifying. /P