knudsen@ihnss.UUCP (01/23/84)
Dan McCabe has a good idea, but it's even simpler than he thought, as far as hardware is concerned. In your plug or cable from the terminal, simply jumper the RD (receive data) to the CD pin. Coco does the level shifting (RS232 to TTL) itself, just as with RD. Now your driver software has to be written to recognize an interrupt when CD goes from a "mark" to a "space" condition; this is the "start bit" at the front of each character received. Come to think of it, I wonder whether Coco OS9 doesn't already do this? Hope not! Shame that Coco lacks a hardware programmable timer (eg, 6522). If she did, you could write a low-overhead software UART that operated only on the *transitions* of the RD (as the above scheme generates). -- mike k