NRT405F901 manual
Chapter 16
Modbus signal types
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Chapter 16 Modbus signal types
Modbus types
Modbus signal types (types listed below):
1 = Coil Status Register (Modbus function = 1, 5 and 15)
2 = Discrete Input (Modbus function = 2)
3 = Coil Status Register (Modbus function = 3, 6 and 16)
4 = Input Register (Modbus function = 4)
Supports the following 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
Scaling factor Modbus
All floating point numbers have a scaling factor of 10. Integers, Index and Logic signals
always have a scaling factor of 1.
Wiring, Modbus
A Modbus type protocol consists of multiple layers (OSI model). The bottom layer is always
the physical layer, the number of connection wires and signal levels. The next layer describes
the communication digits (number of bits, stop bits, parity bits etc). After these come the
layers describing Modbus-specific functions (number of digits per message, the meaning of
different messages, etc). For Modbus, the bottom layer can be either RS485, RS422 or
RS232.
RS485 and RS422
RS485 and RS422 constitute the purely electrical part of the protocol, ie. 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. It may prove necessary to shift A and B in order to make
Modbus work properly. RS485 is a so called half duplex communication: The
communication can only go in one direction, eg. the main unit will initially send a request,
and thereafter listen to the reply. A and B are used both for sending and receiving.
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.
