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Knowledge Base / Protocols / Modbus

Why is the bit or coil level data incorrect?

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Bit read and write commands are issuing properly, and responses are coming from the slave(s), but the data is not correct.

When setting the intaddress field for a bit or coil Modbus command (Modbus function codes 1, 5, and 15), the internal address specified in the command must be at the bit level.

Example:
If the command is issuing a bit read (function code 1), and you wish to place the data in word 600 of the MCM module, the intaddress field should be set to 9600 (600 * 16 bits per word = intaddress of 9600).
This would place the data in word 600, bit 0 of the MCM module. An intaddress of 9601 would place the data in word 600, bit 1.

Second Example:
If the command is issuing a bit read, and you wish to place the data in word 812 of the MCM module, the intaddress field should be set to 12992 (812 * 16 bits per word = intaddress of 12992).
This would place the data in word 812, bit 0 of the mcm module. An intaddress of 12993 would place the data in word 812, bit 1.