1-6
QIn the following example, a Master Device requests an EPM Slave at address 01H to transmit the
value at Register 00256. The Slave replies with an error, indicating that it is busy.
Meaning Hex Meaning Hex
Address 01H Address 01H
Function Code 03H Function Code 83H
Data Starting Address-Hi 01H Error Code 06H
Data Starting Address-Lo 00H CRC-Lo C1H
Number of Registers-Hi 00H CRC-Hi 32H
Number of Registers-Lo 01H
CRC-Lo 85H
CRC-Hi F6H
Master Packet Slave Packet
Table 1.6: Exception Response Example
1.8: Modbus Extensions
QModbus Read Requests have a maximum size when using standard Modbus function. GE devel-
oped Enhanced (Non-Standard) Modbus Read Requests to allow larger than standard responses.
This requires fewer requests and, is therefore, more efficient and total download time is reduced.
This function is also more efficient with Log Retrieval. It allows the Network Card to have DNP
communication with the main unit utilizing a Modbus connection.
The following are non-standard extensions to the Modbus Protocol. EPM devices support the
following additional Modbus Function Codes:
Function Code Description
Hex Dec
23H 35
42H 66
Read Holding Registers Multiple Times
Encapsulated DNP for LAN/WAN
Modbus Extensions