Relay Out
The relay output is an "optoMOS relay", wired to be bi-directional. It is fully isolated, so the +/- indicated on the connector are technically inaccurate. The equivalent circuit, assuming relatively ideal components is:
+-----------------------o Rly Out +
|
o 350V holdoff
software ---- / 120mA max continuous Outside
control o >2kV isolation World
|
max 35ohms <
(note 1) <
|
+-----------------------o Rly Out –
Note1: there is not actually a resistor in the path, but the switch has a maximum internal resistance of 35 ohms.
Trigger in
The trigger input is a transistor circuit set up to sense current between the pins. The +/- really DO matter, and there is no isolation circuitry. The equivalent circuit is:
+-------+
| |
1620 ohms < o current-sense
(note 2) < o ~1mA threshold
| |
----- |
12V DC - +-----------------------o Trig in +
source -----
- +-----------------------o Trig in -
| |
+-------+
Note2: In this case, there really are resistors in the path, to limit the current (to at most 8mA), and to give us something to sense the current across.
The user can connect anything from a wire (basically zero ohms) to a maximum of 5k ohms of resistance between the two terminals to complete the path and trigger the current-sense. (In this ideal world, an ideal switch is really just a wire that's sometimes there, sometimes not there, so that would work fine too.)