8251 troubles

From: cclist at sydex.com> To: cctalk at classiccmp.org> Date: Wed, 30 Apr 2008 10:25:00 -0700> Subject: RE: 8251 troubles> > Date: Tue, 29 Apr 2008 17:49:41 -0700> From: dwight elvey > > > Still a strange problem.> > 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>