[Coco] Serial I/O / Printer port - “printer ready”, etc.

[Allen Huffman]

The Serial I/O (printer) port on the CoCo provides transmit (TX), receive (RX), carrier detect (CD) and ground (GND).

I was aware of BASIC hanging if you tried to print and the printer was not online. I had been assuming this was due to using the CD line to indicate the printer was ready, but according to this 1983 Compute! article, it was the RX line:

https://www.atarimagazines.com/compute/issue37/coco_printer.php

This is good, because I was wondering if I could use RX with my SirSound Jr. project instead of the CD line so folks could build one using a $4 Arduino clone and a .92 cent RS232-to-TTL adapter that did not provide CD.

From an old modem program I wrote, I see this detects carrier:

CD=PEEK(&HFF20)
IF (PEEK(&HFF21) AND 128)=128 THEN carrier detect

And from the 500 POKES, PEEKS and EXECS book in the archive, it shows this for printer ready:

IF (PEEK(&HFF21) AND 1)=0 THEN printer is ready

I don’t have a way to test the printer one, so I ask if someone here could verify this information is correct. I used the CD detect one, so I think it is okay.

Being able to use two sources will enable me to have one indicate “sound playing” and the other to indicate “room in the buffer, you can send more data.”

Fun stuff.

		— A