I’m using the 4 core Ping Cable for carrying RS485 differential signals at 9600 baud in a scuba diving Sat Nav project. I’d like to terminate the cable ends with appropriate sized resistors. To determine this I’m asking here for the characteristic impedance of the ping cable, which would then be the resistor value needed on each end of the cable. I’ve seen 120 Ohms used in some projects but it would be good to know what the value is for this specific cable.
I’ve found the answer to my own question, I’ll include the info here for future reference.
The schematic for the Ping Sonar UART to RS485 adaptor shows an optional 120 Ohm resistor across the A and B differential pair enabled by a jumper and this adaptor is used at both ends of the ping cable, one in the ping hardware case and the other in the ROV enclosure.
If you use your own UART to RS485 adaptor, rather than Blue Robotics, then that won’t most likely include the terminating resistors and adding these to your RS485 adaptors across A and B lines to prevent line reflections can be considered. In my case I am getting checksum errors losing 3% of messages at 9600 baud so I am going to add these resistors to see if there is an improvement.