[GreenKeys] C-64 to drive Mdl. 15 TTY (and probably others..)

[drlegendre .]

Based on what I've learned in the past couple of months, it seems that the
venerable Commodore 64 is a very good candidate for driving teletypes such
as the Mdl. 15 and probably others. Of course, it can't do so directly, but
it does handle a couple major hurdles - the C-64 has the ability to
communicate at what seems to be any (low) arbitrary baud rate using a 5, 6,
7 or 8-bit wordsize.

What's odd, is that (per the C-64 Programmer's Reference Guide) the "user"
set baud rates are not implemented - the guide states as much, and also
claims that the parameters for setting a user baud rate are "for future
expansion"

This might be the case for certain ROM versions, but the feature definitely
works on the three US / NTSC examples I happen to have. So if you want
45.45 baud (and I do), you can have it.. From BASIC, the command syntax to
open a logical file to the RS-232 port looks like this:

OPEN 2,2,0,CHR$(224)+CHR$(0)+CHR$(143)+CHR$(43):GET#2,ZZ$

This should create a port running 5N2 @ 45.45 (5 data bits, no parity, 2
stop bits at 45.45 baud). The "GET#" command is required to initialize the
port - do NOT omit, or everything will go screwy.

(More in a while..)
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