The Kinematics serial converter 2199-801 converters where used on site to talk to modbus ascii & modbus RTU devices (one converter for rtu, another for ascii).

Notes:

1. The converter has built in 120 ohm terminating resistors. Do not place or turn on terminators in the middle of a data loop, put on only those devices at the end. If at all possible use a daisy chain connection as opposed to a star or a line with long spurs.

2. Follow the wiring diagram CAREFULLY as the terminals have a strange wiring order:

Labeled on converter

2    TX+
1    TX-
3    RX+
4    RX-
5    Screen

3. The Screen is an 0V connection so connect to 0V or signal ground at the other end.

If you see the RX light "glow", it means that sufficient noise is causing a signal to appear on the rx lines. Check the OV or screen is connected from the converter to the PLC.

4. If comms is randomly bad, check the modbus addresses of each device is unique as some collisions will occur if two devices are set to the same number (ie data from one plc will be mixed up with another).
Also check modbus addresses if the system is setup for a number of devices and some of those devices are not responding.

5. If the Modbus ASCII driver detects the wiring is reversed it will state the error "Break Received", as it can't decode the characters properly.