How do I connect 2-wire RS-485 to Babel Buster SP?
Pin connections are shown below. RS-485 (EIA-485) is by definition 2-wire. Any reference to 4-wire RS-485 is really EIA-422. Babel Buster SP supports EIA-422, but also automatically supports EIA-485 with a simple wiring configuration. Simply connect all the pluses together, and all the minuses together. Connect RXD+ to TXD+ on Babel Buster SP, and that one connection to + on your Modbus device. Do the same with the minus side. Then set the DIP switches for EIA-485 half duplex, termination ON. Although called "2-wire" you really need 3 conductors, 2 wires for signal + and - with a third conductor for ground/common.
If your Modbus device has its RS-485 terminal marked A and B, be aware that about half of the equipment out there has A and B backwards. This is due to the fact that A and B as published by EIA-485 are the reverse of A and B as defined by the RS-485 chip makers. Somewhere along the line they didn't compare notes. There is no harm done in reversing the wires, it simply will not communicate. If you are having trouble communicating, especially if the device terminals are marked A and B, try reversing them.
The above view is the male DB9 as seen looking into the Babel Buster SP. Important: This is NOT a PC COM port. Study the wiring table below carefully. Also keep in mind that while Pin 1 is on the left on the male connector, Pin 1 will be on the right looking at the female connector.
