Martin,

Is there any specific reason that you want doing things "the old fashioned
way?"

The '55A doesn't have enough I/O pins to interface both to a parallel port
and to an external UART.

An old UART chip like the 16C550 is probably more expensive now than a PIC
with embedded UART. If I remember it right, he PIC16C55A doesn't even have
interrupts. That will require you to write isochronous code and bit-bang
the serial bits.

The voltage level translating is easy, just use a MAX232 or similar.

Cheers,
Isaac

Em qua, 21 de ago de 2019 23:58, Martin McCormick <martin.m@suddenlink.net>
escreveu:

>         I was digging in to my box of now very old PIC's and came
> across some as of yet never-used PIC16c5A's.
>
>         I need to convert the TTL output of an older PC's
> parallel printer port to a serial data stream which will
> ultimately be the DATA input pin on a RS-232 port on another PC.
>
>         I know I will need a level shifter to get the +-12 volts
> but my question is basically can the synchronous  serial device
> in the 16C55a be made to behave like a UART?
>
>         The parallel port on a PC sends an active-low strobe
> pulse when the 8 data lines have a character on them and the data
> are valid when the strobe goes back high.
>
>         I would then want to squirt the data bits at an output
> plus 1 stop bit and then send an ACK back to the PC.
>
>         At that point, the PC strobes if there is a new character
> and the whole process starts anew until there are no more to
> send.
>
>         I don't have any 16550A's but I do have these 16C55a's
> which might work plus provide the logic to handle the strobe
> catching and acknowledging and maybe even Busy signals.
>
>         The PC will temporarily stop hammering the parallel port
> if you bring Busy high.
>
>         There are some other printer-specific pins such as On/Off
> line and no paper indicators but I will probably just hard-wire
> those to ground or a pull-up resistor, whatever they need to
> work.
>
>         My question is would I be better off buying a 16550A UART or
> using the 16c55A PIC running at the right clock speed, of course?
>
>         This is one of those impulse projects that I want to keep
> simple because I didn't think I would need to do it but I
> discovered that I do, in fact need to at least give it a try.
>
>         Many thanks.
>
> Martin McCormick WB5AGZ
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