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
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.