42 Chapter 16 Modbus signal types RCF manual
RS422 is a full duplex communication, meaning that 4 connecting wires are required; 2 for
sending (Tx+ and Tx-) and 2 for receiving (Rx+ and Rx-). Tx is used for sending and Rx for
receiving, meaning the Tx in a unit must be connected to the Rx in another and vice versa.
Pertaining to signal levels, etc., RS422 and RS485 are identical.
In order to connect RS485 and RS422: Connect Tx+ to Rx+ and Tx- to Rx- on the RS422
unit. 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. It is usually easiest to find out what fits where simply by
experimenting. Wrong polarity makes the system not function, but is incapable of harming
any unit.
Tx+ -----|---------------------------------------- A (or B)
|
Rx+ -----|
Tx- -----|---------------------------------------- B (or A)
|
Rx- -----|
Bit rate, two stop bits, parity is next layer.
These settings must correspond to the settings in the main unit. Find out what the settings for
the main unit are, and then enter the same settings into the controller.
Parity can be set to odd, even (FS) or none. If no parity is set, two stop bits will
automatically be used. If odd or even parity is set only one stop bit will be used, or the total
amount of bits will be too great. 1 start bit, 8 data bits, 1 parity bit and 1 stop bit gives a total
of 11 bits, which is the maximum.
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