Modbus Protocol Implementation; Transmission Modes; Address Field; Function Field - Satec PM172P Reference Manual

2 Modbus Protocol Implementation

For detailed information about Modbus protocol, Modbus message framing and error checking,
refer to the "Modicon Modbus Protocol Reference Guide". It can be downloaded from the
www.modbus.org Website. The following paragraphs outline some issues concerning the
implementation of the Modbus protocol in the PM172.

2.1 Transmission Modes

The PM172 can be set up to communicate on a Modbus network using RTU transmission
mode. Refer to the "Series PM172 Powermeters, Installation and Operation Manual" on how
to select the transmission mode in your meter.

2.2 Address Field

The address field contains a user assigned address of the instrument (1-247) on a Modbus
network. Broadcast mode using address 0 is not supported.

2.3 Function Field

The Modbus functions implemented in the PM172 are shown in Table 2-1. Function 04 can
be used in the same context as function 03.
Table 2-1 Modbus Function Codes
Code (decimal)
03
04
06
16
1
08
1
The PM172 supports only diagnostic code 0 - return query data.

2.4 Exception Responses

The instrument sends an exception response when an error is detected in the received
message. To indicate that the response is notification of an error, the high order bit of the
function code is set to 1.
Implemented exception response codes:
01 - Illegal function
02 - Illegal data address
03 - Illegal data value
04 - Device failure
When the character framing, parity, or redundancy check detects a communication error,
processing of the master's request stops. The instrument will not act on or respond to the
message.

2.5 Modbus Register Addresses

The PM172 Modbus registers are numbered in the range of 0 to 65535. From the Modbus
applications, the PM172 Modbus registers can be accessed by simulating holding registers of
the Modicon 584, 884 or 984 Programmable Controller, using a 5-digit "4XXXX" or 6-digit
"4XXXXX" addressing scheme.
Meaning in Modbus
Read holding registers
Read input registers
Preset single register
Preset multiple registers
Loop-back test
Action
Read multiple registers
Read multiple registers
Write single register
Write multiple registers
Communications test
9