Modbus Protocol Implementation; Transmission Modes; Address Field; Function Field - Satec EM720 Reference Manual

2 Modbus Protocol Implementation

For detailed information on the Modbus protocol, message framing and error checking,
refer to the Modbus Protocol Reference Guide. It can be downloaded from the Modbus-
IDA Website at http://www.modbus.org/. The following paragraphs outline some
issues concerning the implementation of the Modbus protocol in the EM720.

2.1 Transmission Modes

The EM720 can be set up to communicate on a serial Modbus network using either
RTU, or ASCII serial transmission mode, and via the Internet using Modbus/TCP mode.
Refer to the EM720 Operation Manual for information on selecting 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.
When communicating via the Internet, the address field is not checked and is returned
in the response message header.

2.3 Function Field

The Modbus functions implemented in the EM720 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 EM720 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 -
02 -
03 -
04 -
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 EM720 Modbus registers are numbered in the range of 0 to 65535. From the
Modbus applications, the EM720 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.
10
Meaning in Modbus
Read holding registers
Read input registers
Preset single register
Preset multiple registers
Loop-back test
Illegal function
Illegal data address
Illegal data value
Device failure
Action
Read multiple registers
Read multiple registers
Write single register
Write multiple registers
Communications test