Electronics Help: Current Loop
Staying late at work today I set about turning this PCB into this schematic. (I also replaced the MC1488 / MC1489 combo with a MAX232, eliminating the need for a bunch of diodes and feeding the board +/-15VDC.)
It supposedly turns RS232 into current loop, where amperage is used to signal 0 or 1 instead of voltage. This makes the signal more robust in noisy environments or with long wire runs.
The thing is, when I look at this, I can’t quite see it as being anything more than a device which takes RS232 serial and converts it to be 0V or 30V (for low or high), and optically isolates data going back to the PC. I don’t see any way it actually varies the amperage while keeping the voltage steady.
Can someone correct me, or confirm what I’m seeing? The schematic, as made from the board, is here.
(Also, I need to remove the 7486 part. Each currently grounded leg of the gate was originally set to a jumper to select if it should be high or low, but the needs of this board are fine having them all grounded. As it’s XOR on all of them, and one leg is always pulled low, the output will always be whatever the input is. So, I think I can eliminate the 7486. I believe the jumpers serve to invert the signal, should it be needed. But it’s not. And I didn’t remove the 7486 while removing the jumpers because I wasn’t certain of it was used for anothing else. But now I know. And knowing is half the battle.)
it’s not a help, but consider downloading the orcad student version and see if you can do a simulation. might help you work it.
I may do that, simply to play with simulations if nothing else. It’s such a simple circuit and I can’t see the amperage being diddled anywhere, so I’m fairly confidant that it’s not what I was told it is.