8251 troubles

[Chuck Guzis]

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