ВШПА.421412.304 И1 SCIENTIFIC-PRODUCTION ENTERPRISE VIBROBIT LLC
Table 10. List of ModBus protocol supported diagnostic commands
Command code Description
0x0000 Echo response
0x0001 ModBus protocol counters resetting and “Listen Only” mode exit
0х0004 Switching on “Listen Only” mode
0x000A ModBus protocol counters resetting
0x000B Transfer number of received messages without errors
0x000C Transfer number of received messages with checksum errors
0x000D Transfer number of received messages with errors (except for checksum errors)
Checksum calculation in messages
CRC checksum consists of two bytes. CRC checksum is calculated by transmission unit and appended to each
message. Receiver calculates checksum during receive and compares with CRC field of received message. CRC
counter is pre-initialized by 0xFF value. Only 8 data bits are used for checksum calculation (start-, stop- and parity bits
are not used for checksum calculation).
Module ModBus protocol control features
Operating parameters and module status register addressing is not aligned by 16-bit words. “Number of
registers” parameter in ModBus commands is indicated in bytes.
During operating parameters and module status writing/reading, data is transferred according to C-based data
storage in memory (low-order bit, then – high-order bit), but not according to ModBus standard requirements.
If odd number of bytes is acquired during reading, response will contain even number of bytes (per unit greater,
than acquired). During writing of odd number of bytes, even number of bytes must always be transferred (per unit
greater, than required), as virtually indicated number of bytes is written to module parameters.
Maximum size of writable/readable bytes in one transaction is 64 bytes.
Note. RS485 bus terminator is provided on MK40 Module board. If the module is last to be connected to
RS485 bus and standard 120 Ohm bus terminator is missing, bridge with bus terminator must be installed on the
module board for normal operation of RS485 interface.
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