Regarding loop resistance

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Dear friends, I would like to ask you all about the issue of circuit resistance in instruments. When I used to communicate with instruments, since the wire ends at the cabinet connectors usually didn’t protrude, I made a custom wire with a resistance of 250 ohms to facilitate wiring. During communication, I would remove a wire from the connector and connect it to my custom wire; then I would attach one end of that wire to the connector, and hang the communicator on either side of the resistor. In this way, I could maintain normal communication regardless of whether there was any loop resistance in the original wiring. I tried many places, and it works in all cases, whether there is a resistor in the original circuit or not. Recently, in two different places, I encountered this issue: it works fine without using my custom cable, as it’s very inconvenient to make connections without cable ends; however, when using my custom cable, the connections are easy to make, but communication becomes impossible. In one case, I changed the resistance of my custom cable to 130 ohms, and then communication was possible. Another thing: I’ve changed it to 8 ohms, but communication is still not possible. I wonder what the resistance value must be in the original circuit. Both locations use control cabinets from Zhejiang University’s Central Control System, and the cards used inside are the same as well. I just checked some information – could it be that the power supply voltage is low, which in turn results in a lower maximum load resistance? When another component is added, communication is no longer possible! ! ! Could such a high resistance value have any impact on the instrument?