Rockwell Automation Publication 5069-UM003A-EN-P - May 2018 59
Configure Compact 5000 I/O Serial Module Chapter 4
Master Command List Limitations
• A maximum of 50 commands can be created. The commands are subject
to available connection memory.
• Each Modbus Master supports up to two data connections.
• Each data connection supports a max of 446 bytes of read data and 482
bytes of write data. For a second connection, the connection supports a
max of 450 bytes of read data.
• Each command can have a max of either 125 words read or 123 words
write or 2000 coils/discrete input read or 1968 coils written.
• Each command uses:
– Two bytes of input data per holding register or input register read.
– One byte of input data per every 1…8 coils or discrete inputs read.
– Two bytes of output data per holding register written.
– One byte of output data per every 1…8 coils written.
• An error message appears when connection memory is exceeded.
Master Command Memory Usage
The following is an example of master command memory usage.
• Maximum single connection configuration for registers:
– 223 words * 2 bytes/word read = 446 input bytes
– 241 words * 2 bytes/word written = 482 output bytes
• Maximum single connection configuration for coils:
– 3576 bits / 8 bits/byte read = 447 input bytes
– 3856 bits / 8 bits/byte = 482 output bytes
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