APS 6RL Series, Operation Manual

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6RL SERIES OPERATION MANUAL
SECTION 7: REMOTE PROGRAMMING
Entire Contents Copyright

2018 by Adaptive Power Systems, Inc. (APS) • All Rights Reserved • No reproduction without written authorization from APS.
6RL Series Regenerative DC Load Operation Manual Page 131 of 204
7.10 ModBus TCP Details
ModBus TCP is based upon ModBus RTU and this section is only about the differences. The
core of a ModBus TCP message is ModBus RTU. Refer to section 7.9, “ModBus RTU Details”
for more information. Differences in ModBus TCP compared to ModBus RTU are:

The message requires an additional 6 bytes long MBAP header

The checksum is omitted

It can only be sent via port 502 and only with ModBus TCP interface modules. The
port is reserved and must thus not be adjusted in the setup MENU of your unit
where you define the port for standard TCP connections.
As a result, Modus TCP messages are always 4 bytes longer than ModBus RTU messages
(byte 0 of the RTU message is replaced by the actual 7th byte of the additional MBAP
header).
The MBAP header is specified like this:
Bytes Meaning Explanation
0 + 1 Transaction identifier This identifies the message. It is copied by the unit in the response and is
used to identify a certain message in a pool of incoming transmissions if
multiple units are communicating with the PC and the response is not
immediately. The identifier is an arbitrary value between 0 and 65535.
2 + 3 Protocol identifier Here always 0 = ModBus protocol
4 + 5 Length Number of remaining bytes in the message, i.e. the length of the ModBus
RTU core message minus 2.
7.10.1 Example of ModBus TCP messages
The example is a query for reading the unit’s nominal voltage. Reads holding registers
extended by the MBAP Header (arbitrary transaction identifier 0x4711 used):
Query Message
MBAP header Addr FC Start Data
0x4711 0x0000 0x0006 0x00 0x03 0x0079 0x0002
Possible Response
MBAP header Addr FC Length Data
0x4711 0x0000 0x0007 0x00 0x03 0x04 0x42A00000
The response contains a floating point value in “Data”, which translates to 80(V).