9
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
Website. The following paragraphs outline some issues
concerning the implementation of the Modbus protocol in the PM130.
2.1 Transmission Modes
The PM130 can be set up to communicate on a Modbus network using RTU transmission
mode. Refer to the "Series PM130 PLUS 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 PM130 are shown in Table 2-1. Function 04 can
be used in the same context as function 03.
Table 2-2 Modbus Function Codes
Code (decimal)
Meaning in Modbus
Action
03
Read holding registers
Read multiple registers
04
Read input registers
Read multiple registers
06
Preset single register
Write single register
16
Preset multiple registers
Write multiple registers
08
1
Loop-back test
Communications test
1
The PM130 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 Transaction Timing
The PM130 response time to master requests is indicated in Table 2-2.
Table 2-2 Response Time
Response Time, ms
Baud Rate, bps
Min Max
Typical
9600 13 15 13
19200 11 12 11
57600 9 10 9
115200
9 10 9
Provided
by
Northeast
Power
Systems,
Inc.
www.nepsi.com
