🚀 go-pugleaf

>>>>> "R" == Robert R Radina <radina@vulcan.inlink.com> writes:

R> ktra93@control.auc.dk (K Xoeterix) wrote:
R> : I would like to connect my PC to my Commodore 64 through a RS232-link.
R> : Has anybody tried this before? Is there anything weird I should
R> : know about doing this? For instance, what voltage levels does the
R> : PC's serial port use (CBM64 uses TTL) ?

R> I didn't know the CBM64 had a serial port.  I know it has the "serial"
R> disk-drive/printer daisy chain port, but I don't think this is RS232

The RS232 is voltage level standard - nominal levels are -12 and +12V,
but it is allowed to use -24..-3 as negative, +3..+24 as positive level,
voltages in range -3..+3 aren't defined (there should be some switching
levels somewhere in this range, i.e. some voltage V1<+3V causes signal
to be recognized as positive, some voltage V0>-3V causes signal to be
recognized as negative, V0<V1, within the V0..V1 range signal is recog-
nized as last valid signal detected), voltages outside -24..+24 can
cause damage of hardware. Also, the standard limits input current to
something like 1mA and output short-circuit current to 500mA.

The voltage level translators used in PC can recognize TTL levels on
input, depending on connection of one pin. On output, they give near
+/-12V - make the inputs you connect there have some protection.

The Commodore may have something less sophisticated than PC's 8250
chip - seems it has 8-bit shift register. Note the 8250 recognizes
sequence consisting of: start bit (which is 1 bit-time of level 0),
data bits (5 to 8, depending on settings, lowest bit sent first),
optional parity bit (odd parity means sum of all data bits and the
parity bit is odd, even parity means the sum is even, it can be set
to be 0 or 1 on every byte regardless of data, if its value is bad
it is detected as parity error), stop bit (which is level 1 during
at least 1 bit-time, level 0 at the time is recognized as framing
error; on output, stop bit can be programmed to be at least 2 bit-
times and it can last more if there is no data to send); the
recognizing is started by initial 1->0 input state transition and
then state is checked after 1/2,3/2,5/2... bit-times from the
initial transition (levels should change at 1,2,3... bit-times).

Maybe it can be simpler to adapt PC's printer port - it has 4
status lines (port base+1, TTL-LS inputs, base can be found at
40h:8..0Fh), 5 control lines (port base+2, TTL-LS inputs and OC
outputs) and 8 data lines (port base+0, TTL-LS 3-state outputs
usually w/o ability to disable unless you make changes on board,
TTL-LS inputs), note some status/control lines are inverting.