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56 Part IV Signals NRT407 manual
Chapter 16 Modbus signal types
Modbus Type The Modbus types of the signals (types in the list below):
1 = Coil Status Register (Modbus function = 1, 5 and 15)
2 = Discrete Input (Modbus function = 2)
3 = Holding Register (Modbus function = 3, 6 and 16)
4 = Input Register (Modbus function = 4)
Supported Modbus functions:
1 = Read Coils
2 = Read Discrete Input
3 = Read Holding Register
4 = Read Input Register
5 = Write Single Coil
6 = Write Single Register
15 = Write Multiple Coils
16 = Write Multiple Registers
Scale factor Modbus Integer, Index and Logic always have scale factor 1.
Modbus wiring etc. A protocol such as Modbus consists of several layers (OSI-model). The bottom layer
is always the physical layer, number of wires and signal levels. The next layer
describes the communication digits (number of data bits, stop-bits, parity etc). Then
come the layers describing the Modbus specific functions (number of digits per
message, the meaning of different messages etc).
For Modbus, the bottom layer can be RS485, RS422 or RS232.
RS485 contra RS422 RS485 and RS422 are the electric part of the protocol, i. e. the physical layer.
RS485 has two connections, A and B. Often there is also a protective earth. RS485 units are
connected A
A and B B. You may have to shift A and B in order for Modbus to work.
RS485 is so called half duplex communication: Communication can only go in one direction
at a time; i. e. the master will first send an enquiry and will thereafter listen for the reply. A
and B are used for both transmission and reception.
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)