Can you provide additional details on the WVR61xx/WVR71xx Remote Front Panel interface?
The cable carries RS232, +5V power, and two special purpose lines. The +5 V power is the limiting factor in cable length, and the factor that drives the need for 26 gauge wire and a cable shield (if there's no other common ground). Customer-built cables must meet the requirements implied by description of pins 1, 8 and 9 below.
Cable Description for Manual
Connectors (each end): 9-pin D-SUB, male, with jackscrews
Cable: nine conductors, 26 gauge or heavier, shielded
Construction: Wire pin 1 to pin 1 etc, all nine pins. Shield or drain wire bonded to metal shell at each end.
Maximum length: 100 ft
More technical details that are not in the manual:
All signals pass between RS232 receivers and transmitters with typical +- 10v swing. Signal integrity is not likely to be a problem. Power and ground connections may require special attention, though.
+5V power to the remote panel, poly-fused at 3A. Two wires are used to reduce voltage drop. Typical max current is ~900 mA for a few seconds at first turn-on when all LEDs light. Typical draw after is < 200 mA. Remote panel chassis has boost regulator to get from 3V back to 5V.
Ground (chassis). System depends on this wire plus cable shield to return +5 power with low V drop. Total V drop in power and gnd must be less than 2 V at 900 mA (leaving 3 V for remote panel) for operation. This power ground is also chassis ground for both units, so incidental connection, say between rack grounds, can help or hurt. Helps if there is no gnd potential difference by providing additional power return path. Ground potential difference can hurt since it may reduce the apparent "5V" power at the remote unit.
Pins 5, 7
TXD and RTS driven by main box by MAX238 RS232 output, received at Remote panel by another MAX238.
RXD driven by remote panel MAX238, received at main unit.
Remote presence. Driven high by the remote panel to flag the main unit that the panel is connected. Causes main unit to use input from the remote unit instead of the local FP.
Remote Program. Driven by main unit. Flags remote panel to go into re-programming mode for possible firmware update. Hasn't been used to-date, no known plans to use it.
Remote Fault. Driven by main unit. Lights the red FAULT LED on the remote panel in case of main unit fault.
Note: You must cycle power to initialize operation from the RFP module. In general, anytime you connect the RFP module to the instrument, power it down, connect the RFP module, and then power up the instrument.
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