68
Chapter 16
Modbus signal types
Regio Midi manual
RS422 is a full duplex communication which means you need 4 wires, 2 for transmit
(Tx+ and Tx-) and 2 for receive (Rx+ and Rx-). Tx is used to transmit and Rx to receive, which
means that Tx in one unit must be connected to Rx in the other and vice versa. As for signal
levels, etc., RS422 and RS485 are identical.
To interconnect RS485 and RS422: On the RS422 unit connect Tx+ with Rx+ and Tx- with
Rx-. We have now changed a 4-wire system to a 2-wire system and can connect them to A and
B on the RS485 unit. Which goes where is something you most often need to find out by trial
and error. Incorrect polarity will just give nonfunction but cannot harm either unit.
Tx+ -----|---------------------------------------- A (or B)
|
Rx+ -----|
Tx- -----|---------------------------------------- B (or A)
|
Rx- -----|
Bitrate, two stop bits, parity is the next layer
These settings must correspond to the settings in the master unit. Find out how the master is
set and then give the Controller the same settings.
Parity can be set to odd, even (FS) or none. If none is chosen, two stop bits will automatically
be used. It is also possible to configure no parity using one stop bit. In this case, automatic
switching between EXOline and Modbus will not function. If odd or even is chosen, only one
stop-bit is used, or there will be too many bits altogether: 1 start-bit, 8 data-bits, 1 parity-bit
and 1 stop-bit give a total of 11 bits which is the maximum amount.