Several things can be happening.
1) There could be a misconfiguration issue. We checked the scaling configuration and only 0-20mA is available. Tomorrow I take a screen shot of the scaling configuration from the 4-20mA devices that is working on the same AI card. They are configured for volts inputs, and have 250 ohms across the inputs.
2) The maximum 750 ohms referred to is a maximum loop value. Your 4-20mA analog input probably wants a 100 ohm or a 250 ohm resistor across the input to create an IR voltage drop. You need to check the wiring specs for the AI card and see if the 4-20mA dropping resistor is 'built-in' or whether it is a separate resistor that you wire across the terminals. I will check for this.
3) Off-scale, up-scale is a common failure mode indication, which can happen from a multitude of different issues, like no current/voltage 'seen' at the input.
4) The scanner device is effectively a single ended 3 wire, active 4-20mA output that shares a common/ground with the 24Vdc power supply. This is correct
If both of your scanners are using different power supplies powered from different AC circuits, you could have excessive common mode voltage driving the signal off scale up scale.
The scanner's requirement for 'isolated analog inputs' is to avoid the noise and common mode issues associated with grounding, particularly between different scanners. Both scanners use the same 24Vdc power supply
Troubleshooting:
1) You need to simulate 4-20mA from a signal source/calibrator or power supply and resistance network to see what you need to do to get your AI read any 4-20mA signal. We will try this tomorrow. We can drive the 4-20mA with 744 communicator
Disconnect both scanners and preferably use a battery powered 'source' so that common mode is not an issue.
Your card is well marked as non-isolated so it probably doesn't tolerate too much common mode. Even a series resistor network can common mode, depending on where you connect in the series.
2) Once you get the AI reading some reasonable 4-20mA value, connect one scanner to that AI and see what you get.
If it works OK but then goes off-scale upscale when you connect the 2nd scanner signal, then you'll need to get a 4-20mA loop isolator module to isolate the 2nd signal.
