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Simple RS-485 Wiring Question
I'm having trouble getting MODBUS RTU comms working between my laptop and a piece of equipment, via RS-485 half duplex. See description below.

The piece of equipment says the following in the manual for this connection.


Equipment DTE/DCE
TxA -----O-------------------O-------- RxA
| | R
TxB -----+-O-----------------+--O----- RxB
| | | |
RxA -----/ | | \----- TxA
R | |
RxB -------/ \-------- TxB
GND ---------------------------------- GND

DB9 pinouts are
Signal Pin
RxA 2
RxB 8
TxA 3
TxB 7
GND 5

My interface has D- & D+ connection. I only want to go a very short distance so I was hoping I wouldn't require the termination resistors. I have a female db9 connector on a ribbon cable to fit into the devices's male connector. What is the easiest way to screw the ribbon cable wires into the interface's D- & D+ connection to make communications work?

Wow, nice ASCII art.

1. Jumper TxA and RxA; TxB and RxB. No you've got 2 wire half duplex that will connect to D+ & D-

2. A (TxA/RxA) should be (-) and connect to D(-); TxB/RxB should be (+) and connect to D(+).

3. Be aware that vendors do not always label the driver lines the same way. Reversed driver lines do not damage the drivers, but reversing won't work. If labeled opposite, you have to reverse the connections on one end to make it work.

4. At short distance and at 9600 or 19.2K baud you shouldn't need a terminating resistor. I bench test all the time without a terminating resistor.

5. Buy a straight-through serial cable with DB9 connectors and cut off the end that doesn't fit on device. Strip the wires and connect to the D+ and D- terminals.

Wire both pin 2 (RxA) and Pin 3 (TxA) D(-). Same with pins 8 and 7, wire both to D(+).