Sorry to reply to my own post here, I had a bit more time to screw with
this this evening. Turns out it was a programming error (actually a
program entry error :)) that was (apparently) causing the 40mA current
instead of 20mA -- the little "echo" program I coded up wasn't properly
waiting for data to come in before echoing it back, so the 8/L was
constantly sending 377 back across the loop, which I'm assuming accounts
for the difference?
With this corrected, the loop is registering ~23mA idle, dropping to
19-20mA when the PC attempts to send data. Can anyone comment on
whether this sounds reasonable? Anyone have any advice? (Getting
desperate here :)).
Thanks,
Josh
On 9/14/2013 5:19 PM, Josh Dersch wrote:
Hi all --
I'm attempting to get my 8/L talking to the outside world. It's
outfitted with the typical Teletype interface, which I'm attempting to
connect to a PC via acurrent loop to RS232 adapter.
Specifically, I've got one of these:
http://www.bb-elec.com/Products/Serial-Connectivity/Serial-Converters/Curre….
At the moment I have it configured (per the manual) assuming the 8/L
is an active loop device (i.e. the converter's transmit lines connect
to the 8/L's receive lines, and the 8/L's transmit lines connect to
the converter's receive lines). With this configuration I'm able to
get the 8/L to send data to the PC successfully.
I'm not having much luck getting the8/L to receive anything from the
PC, however. If I put my multimeter in the loop, I see a current of
about 40mA, which drops to between 30 and 37mA when the PC is sending
data (depending on what data's being sent). However, I'm not getting
much of anything onpin "E" of the W076 (Teletype Connector) board --
this is supposed to be the "Logic Output" and I'm assuming that should
be the TTL-level data signal, correct?
I do see a very, very tiny signal (in the millivoltrange) on pin "E"
that matches up with the data being sent from the PC, but it's
obviously nowhere near the correct level. I've gone over the W076
board and checked the related diodes and resistors for anything
open/shorted. And I actually did find a bad diode (fell apart when I
desoldered one leg to test) but replacing it made no apparent difference.
At this point I'm wondering if the loop configuration I have is
incorrect. I thought this was supposed to be a 20mA loop, but I'm
getting 40mA. I know very little about the fundamentals of current
loop stuff, so I could be way off here.
Anyone have any advice?
Thanks,
Josh