22 Sep
2013
22 Sep
'13
2:53 a.m.
On 2013 Sep 21, at 10:40 PM, dwight elvey wrote:
I'm looking at the converter schematic but it is
onpage 37?If it is
a 8L, I figure 20 mA.It is -30volts to +5V or 35V.R1 and R2 are 1K
and 750 or 1750 ohms.35/1750 = .02 amps or 20mA. Point N is pulled
to -10v when there is 20mAflowing.One could clamp point P to -10V
with a 10 voltzener to ground. That would still get the 20mAwhen
the contacts were close but limit the voltageacross the contacts
( RS232 converter ) to 15Vinstead of 35V.A half watt zener would
work here as it is only0.2Watts to deal with.I hope this makes
sense.Dwight
(Dwight, just for clarity, I'm referring to the the RS-232->current
loop dongle as the converter, the W076 interface card being on page
37 in the 8/L schematic.)
Some more of the reasoning from the earlier examination of the
schematic:
- When the loop (3,4) is open: pin-4 is pulled up to +5 through R2
(750), pin-3 is pulled down to -30 through R1(1K) and D2 *if the -30
supply is connected to the W076*. However if the -30 is not
connected, pin 4 is (should be) pulled down to -15 through now-
conducting D3 and D7. (D3 and D7 are reverse biased/off if -30 is
connected).
- When the loop is closed (3,4 shorted), -V (15 or 30) has two paths
to flow: to +5 through R2 and to ground through R4 (750) and D4 (D4
now forward biased and clamping the negative V (intended so point E
to the TTL logic doesn't go too negative).
I get the 43mA from the sum of those two paths (25mA to +5 plus 18mA
to GND) when -30 is disconnected. From my calcs, when the -30 is
connected the current is actually lower, despite the higher V,
because of the higher impedance through R1(1K).
-
Josh, the measurements you presented make sense for the -30-connected
case, but not for the not-connected case. They suggest that the -15
is not present. Check whether -15 is supplied to the card (I expect
it is as IIRC you said the 8->PC direction was working, should be
point B but note doc also mentions point K).
Without the -30, -15 should be making it through D3 and D7 to pin-3
(to test, might need a slight load to conduct when open-loop). You
said you replaced a broken diode earlier, might the new diode be
installed backwards? If it's D3 or D7 it would explain the lack of
-15 at pin-3 and would also (counter-intuitively as per above)
explain the reduction in the current (~40mA down to 23).
I think you should figure out what's going on with the W076 before
thinking about modifying the converter. Should try to get the
expected behaviour and measurements out of the W076 without the
converter involved. If that resolves as calculated, the lower V of
20V ( =5-(-15) ) may still be pushing it for the 6N135 but it may work.