22 Sep
2013
22 Sep
'13
12:19 a.m.
On 9/21/2013 3:41 PM, Brent Hilpert wrote:
I'm looking at the RS-232 converter schematic:
http://www.bb-elec.com/Products/Datasheets/r2_Website-Only_0812DS.pdf
and this PDP-8/L schematic:
http://bitsavers.trailing-edge.com/pdf/dec/pdp8/pdp8l/DEC-8L-HR2A-D_8Lschem…
power supply: page 25
W076: page 37
and
http://www.fairchildsemi.com/ds/6N/6N136.pdf
(So all of the following is derived from examining the schematics, not
previous measurements. I had to redraw the W076 diagram so it makes a
little more sense.)
One thing you might check is whether the 'optional' -30V is present on
the W076, it appears it can be supplied internally or externally. The
power supply schematic indicates it is produced by the 8/L supply,
whether it makes it to the W076 will require examination. With the
-30V the 'open-state' loop voltage would be +5-(-30)=35V, which
exceeds the max specs for the 6N135 opto-isolator in the converter.
Could be sending it into breakdown/conduction or damaged the converter.
The -30V supply does seem to be wired to the W076 in my system at Pin
V. If I'm understanding the intended usage here, I think -30V either
needs to be provided by the backplane, or it needs to be fed in
externally through the "-30V" lug. (So it's optional only in terms of
where it's supplied.)
Try removing the -30 if it was present. Without the -30 it appears the
W076 loop voltage should be +5-(-15)=20V (diode switching of supply)
which the converter might be happier with. The converter design
appears to me to be a little on the weak side for V specs.
I disconnected the -30V supply (with a bit of carefully applied
electrical tape to pin V on the edge connector) and with it
disconnected, the loop current stays at 0mA all the time.
If the -30V was present, you might try checking the converter
disconnected from the PDP-8.
- Connect T- to ground, and T+ to a V+=(+5 or +12) through a
1K-or-so resistor.
- T+ should show ~ +0.2V/V+ switching with the RS-232 signal-in.
Another suggestion could be to confirm the W076 is working properly.
If I have things more or less right, without the -30 supply and the
loop disconnected:
- W076 with pins 3,4 open (no external connections to 3,4, measured
relative to the PDP-8 ground):
- pin-4 should = ~ +5V
With or without -30V connected, I'm getting 4.6V.
- pin-3 should = ~ -15V
With -30V connected, I get -34.6V, with it disconnected I get nothing.
- pin-E should = ~ +5V
With or without -30V connected, I get 4.5V.
- W076 with pins 3,4 shorted (and no other connections to 3,4):
- pins-3,4 should = ~ -(15-2*0.6) = ~ -13.8V
With -30V connected, I get -6.8V, without it's 4.5V
- pin-E should = ~ -0.6V (one conducting diode
drop)
With -30V, this is 0.6V, without it's 4.5V.
It looks like the receive-loop current from the W076 will be around 43
mA (a current meter between pins 3-4 for example) which seems odd, but
that's what my calcs say, unless I've misinterpreted something in the
schematic. But it matches what you've been seeing doesn't it?
I'm seeing ~23.5mA at idle (no data being sent), and 20-21mA with the PC
sending a "break." For a short time, I was seeing ~40mA but I
attributed it to my "echo" test program continuously sending data back
from the 8/L instead of waiting for keyboard input
(which would still
indicate a problem). However, I have been unable to reproduce
this
again, no matter what the 8/L sends, the current is right in the range
of ~23mA.
So, given this, does it seem reasonable to wager that this particular
converter's just not up to dealing with the 8/L's transmit loop
voltages? Maybe I could replace the 6N135 with something a bit beefier?
Thanks for the assistance!
- Josh