8251 troubles

Date: Tue, 29 Apr 2008 17:49:41 -0700 From: dwight elvey This is past the point where I'd have my logic probe out (if you don't have one, they're easy enough to make)--and this is a perfect application for one, as the signals aren't super-fast. Bring your test up, send a character to get the software into the state where it quits receiving. Let's check the static signals first. Is pin 22 (DSR/) low? If not, check your "null modem" cable and any level conversion logic upstream from that pin. While you're at it, check that pin 23 (RTS/) is low so that you can transmit when the time comes. Now for the dynamic tests. A logic probe with a "pulse stretcher" is needed here. Fortunately, most have this feature. Next, put a probe on pin 3 (RxD) and make sure that it's pulsing with each character sent. And then check pin 14 (RxRDY). Does it pulse high with each received character? If it goes high with the first and stays high, then you're not retrieving characters after the first. But if it pulses high only once and then stays low, check your chode. Somehow, you're disabling the receiver, perhaps as part of the "TOUT" code. (This is bit 2 of the command word and must be set for the receive side to operate). The third possibility, that RxRDY pulses with each received character, says that you're actually picking up the data and it's not getting echoed back (hint: check the status of your CTS line). Let us know what you find. Best, Chuck