APS 6RL Series, Operation Manual

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6RL SERIES OPERATION MANUAL
SECTION 7: REMOTE PROGRAMMING
Entire Contents Copyright
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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 125 of 204
Note: The bytes in a ModBus message are read from left to right (big endian format),
except for the 16 bit ModBus checksum where low byte and high byte are switched.
7.9.1 Control Message (write)
When sending a status, a value, multiple values or a text, the data part of the message
requires to define at least the target register and one value to write. The protocol checks
the message only regarding the max. length of the register. After the data part, the
checksum is expected. So in case the data part would only contain the minimum two bytes
and thus the message would fulfil the protocol requirements for the selected function code,
the checksum would be expected at the position of the 7th byte. If there were further data
bytes at that position or zeros and the checksum would be at a different position in the
message, the unit would return an error. Hence, the unit will return an error, no matter if
the telegram is too short or too long, because the checksum is wrong. For message
examples see section 7.9.5, “Examples of ModBus RTU messages” on page 128.
WRITE Single Register:
Byte 0 Byte 1 Bytes 2+3 Bytes 4+5 Bytes 6+7
Addr FC Start reg. Data word CRC
0x00 0x06 0...65535 Value to write Checksum ModBus-CRC16
(1
WRITE Multiple Registers:
Byte 0 Byte 1 Bytes 2+3 Bytes 4+5 Byte 6 Bytes 7-253 Last 2 Bytes
Addr FC Start reg. Number Count Data bytes CRC
0x00 0x10 0...65535 0...123 Number*2 n values to write Checksum ModBus-CRC16 (1
Write Single Coil:
Byte 0 Byte 1 Bytes 2+3 Bytes 4+5 Bytes 6+7
Addr FC Register Data word CRC
0x00 0x05 0...65535 0x0000 (FALSE) or 0xFF00 (TRUE) Checksum ModBus-CRC16
(1
1
Note: The entire 16 bits of the data word represents 1 coil only, for writing and reading.
7.9.2 Response Message (read)
A response from the unit is usually expected after a query or if something has been set and
the unit confirms the execution.
Expected response for WRITE Single Register:
Byte 0 Byte 1 Bytes 2+3 Bytes 4+5 Bytes 6+7
Addr FC Start reg. Data CRC
0x00 0x06 0...65535 Written value echoed Checksum ModBus-CRC16 (1
Expected response for WRITE Single Coil:
1 See ModBus Checksum