RCF manual Chapter 16 Modbus signal types 41
Chapter 16 Modbus signal types
EXOL types EXOL signal types:
R = Floating point number (Real) (-3.3E38 - 3.3E38)
I = Whole number (Integer) (-32768 - 32767)
X = Index (0 - 255)
L = Logic (Logic) (0/1)
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.
EXOline/Modbus The RCF controller will automatically switch between EXOline and Modbus, depending on
what type of communication is used. This switch-over will take place without any errors in
communication resulting. The exception is when communicating via Modbus with a
configuration of 8 bits, no parity and 1 stop bit, in which case the switch must be made
manually.
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 (N on EXO
controllers). 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.
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