Instruction Manual

6.4.12 PCS MODBUS MASTER COMMANDS
The PCS Modbus RTU interface application uses Initialization, Read, and Write Modbus messages
to communicate with the PLC. During InItialization, the PCS sends a Restart Communications Op-
tion function to the PLC. The Read Modbus message causes data (L-values, C-values, or L- and
C-values) to be sent from the Modbus PLC to the PCS master, and the Write Modbus message
causes data (L-values, C-values, or L- and C-values) to be sent from the PCS master to the Mod-
bus PLC. The Modbus command set that is used for the Modbus messages is listed in Table 6-1
as follows:
Table 6-1. PCS Modbus Master Commands
Function
Code
Data
Type
Title Description
1 L Read Coil [Output] Status This command obtains the current status
(ON/OFF states) of a group of logic coils (L-
values).
2 L Read Input Status This command obtains the current status
(ON/OFF states) of a group of discrete inputs
(L-values).
3 L and
C
Read Holding Registers This command obtains the current binary value
contained in one or more holding registers (C-
values). It can also be used to obtain L-values
stored in other PLC memory segments;
however, those L-values must first be moved
by the user into the lower memory addresses
(top) of the holding register memory segment
before the L- and C-values are read from the
memory.
4 L and
C
Read Input Registers This command obtains the current binary value
contained in one or more input registers (C-
values). It can also be used to obtain L-values
stored in other PLC memory segments;
however, those L-values must first be moved
by the user into the lower memory addresses
(top) of the input register memory segment
before the L- and C-values are read from the
memory.
5 L Preset [Write] a Single Coil This command causes the last bit of the L-
memory area (DDI-A - L2047, DDI-B - L1535)
to be written to a single coil in the PLC.
6 C Preset [Write] Single Holding
Register
This command places a binary value into a
holding register.
8 N/A Loopback Diagnostic Test This command provides an assortment of
Diagnostic Code functions to help evaluate
Modbus communications processing. It is
described in greater detail in Section 6.4.12.
15 L Force [Write] Multiple Coils This command forces a series of consecutive
logic coils to specified ON or OFF states.
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53MC9015 53MC5000 PLC and Printer Interfaces
6-6
MODBUS2