18 Sep
2013
18 Sep
'13
8:08 p.m.
On 9/18/2013 7:09 AM, Ethan Dicks wrote:
From: Josh Dersch <derschjo at gmail.com>, Date:
Tue, 17 Sep 2013 21:09:46 -0700
> 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?
Thanks to all for the replies, I'm going to attempt to condense my
further answers / questions into this response to cut down on the amount
of mail in all of your mailboxes :).
Do you have a picture of your setup you can share?
I'm wondering what the
other end of the wiring coming of your W076 card looks like.
This style of Molex connector was rather common (in the DEC world
for sure)
http://www.vintagecomputer.net/digital/PDP11-05/M9970/wires_fr_1105_to_TTY_…
My 8/L came with its W076 directly wired to the terminals on the ASR33
that came with the system. I've since wired up my own set of connectors
(using some 9-pin Mate 'n Lock connectors I happened to have on hand).
The wiring from the W076 (using the W076's labeling) to the Current
Loop->RS232 adapter is:
3 -> T-
4 -> T+
6 -> R-
7 -> R+
Relay +/- : Not connected currently
From your subject line, it sounds like you have a 20mA<->RS232
adapter. If so, what model and how is it wired to your W076?
I've covered this before, but just for reference, It's one of these:
http://www.bb-elec.com/Products/Serial-Connectivity/Serial-Converters/Curre…
(the 9-pin serial variety). I have it wired up as in Fig. 2 of the
manual linked to from that page, using the wiring described above.
Tony Duell wrote:
I would then be inclinend to fidn some whay of
cotnrolleing the TxD RS232
input ot the interface. Either by using the 'break' bit in the PC;s
serial chip or by disocnnecting that line from the PCB and using jumpers
to pull it high or low. WIth it low (-ve votlage), there should be 20mA
or some fliwiugn round the loop. 23mA is fine. WIth it high (+ve
voltage), there should be essentially no current flowing in the loop. If
you don't get this there is a hardware problem.
I just tested this out with the PC sending a "break" (for the record,
I'm using a *real* RS-232 port, not a USB->Serial adapter for this -- I
recently discovered that my USB adapter doesn't actually support 110
baud, guess how I found that out? :) ). When sending a break, the
current only drops to about 19.5mA.
So, I guess there could be a hardware problem here, but this assumes
that the way I have the 8/L hooked up to the adapter is actually
correct. I had thought that the configuration I have is correct (I had
thought that the 8/L would be considered an "active" port) but I'll
reiterate that I've never fooled with current loops before and so my
assumption may be in error. I probably have a lot to learn here :).
Thanks again for the help...
- Josh
-ethan