Programmable Serial-to-Ethernet Device Server
PDS Series User Manual, Version 2.5, Mar. 2019
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Leading 6 bytes of Modbus/TCP protocol:
Byte 00
Byte 01
Byte 02
Byte 03
Byte 04
Byte 05
Transaction identifier
Protocol identifier
Length field (upper
byte )
Length field (lower
byte)
Transaction identifier:
Assigned by Modbus/TCP master (client)
Protocol identifier:
0
Length field (upper byte):
0 (since all messages are smaller than 256)
Length field (lower byte):
Number of following RTU data bytes
RTU Data Structure
Byte 06
Byte 07
Byte 08-09
Byte 10-11
Net ID
(Station number)
Function Code
Data Field
Reference number
(Address Mapping)
Number of points
Net ID:
specifies the address of the receiver (Modbus/TCP slave).
The first byte in the Modbus structure of the Modbus RTU is the receiver’s address. The Valid
addresses are in the range of 0 to 247. Addresses 0 is used for broadcast, while addresses 1 to
247 are given to individual Modbus devices.
The Net ID should be assigned to the address for the target device you want to read from and write
to. For example, if you want to use the built-in I/O of the PPDS, assign the Net ID to the PPDS's ID; if
the PPDS is used as a Modbus/TCP to Modbus/RTU gateway, assign the Net ID to the address for
the RTU slave device.
