Can you describe the current loop output from the PLC? I'd expect two terminals, probably + and -, or a single output and ground. Can you draw a wiring diagram from the PLC to the serial interface?
Presumably the current loop output is using something like 4mA for one digital state and 20mA for the other? i.e. no analog signalling.
You could disconnect the printer and hook up a resistor (ideally 250Ω) across the PLC current loop output and measure the voltage across it using an oscilloscope. You should see it flicking between 1V and 5V and building up a digital trace of the binary value of characters sent.
Is J3 open or closed? Is J4 open or closed? I can't read the designation of the next jumpers in the yellow path, but looks like J6, with JRS above. Is J6 open or closed?
Actually, could you provide a list of jumper settings you are using please?
I don't think you need the 12V supply. I think that's just for generating the +/- 12V signals for output. You do need the 5V supply as that is running the logic circuits.
I suspect you don't need GND and FG connected to the PLC. You have a current loop which is isolated from the printer by way of the optoisolator, so a common ground is not needed. To be honest, the 47Ω sounds like a cheap hack to get the LED in the optoisolator to turn off properly or something. It's a current loop, so everything in the loop experiences the same current, from 4 to 20mA. 4mA is sufficient to light the LED, and so is 20mA, meaning it would never turn off, so you'd never get a stream of bits.
Summary: please show wiring diagram from PLC to this interface, and list of jumper settings.
Presumably the current loop output is using something like 4mA for one digital state and 20mA for the other? i.e. no analog signalling.
You could disconnect the printer and hook up a resistor (ideally 250Ω) across the PLC current loop output and measure the voltage across it using an oscilloscope. You should see it flicking between 1V and 5V and building up a digital trace of the binary value of characters sent.
Is J3 open or closed? Is J4 open or closed? I can't read the designation of the next jumpers in the yellow path, but looks like J6, with JRS above. Is J6 open or closed?
Actually, could you provide a list of jumper settings you are using please?
I don't think you need the 12V supply. I think that's just for generating the +/- 12V signals for output. You do need the 5V supply as that is running the logic circuits.
I suspect you don't need GND and FG connected to the PLC. You have a current loop which is isolated from the printer by way of the optoisolator, so a common ground is not needed. To be honest, the 47Ω sounds like a cheap hack to get the LED in the optoisolator to turn off properly or something. It's a current loop, so everything in the loop experiences the same current, from 4 to 20mA. 4mA is sufficient to light the LED, and so is 20mA, meaning it would never turn off, so you'd never get a stream of bits.
Summary: please show wiring diagram from PLC to this interface, and list of jumper settings.
Statistics: Posted by ame — Tue Jul 29, 2025 10:06 pm