BEL constant in MAINDEC-8E-D0AB-PB

[Pontus Pihlgren]

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