Protech Systems SI-W509 User Manual page 38

Error Response
Offset Description
00 Slave ID address
01 Function Code
02 Data
03 ~ 04 CRC
SI-W509 SERIES USER MANUAL
Chapter 4 MODBUS RTU Protocol
Counts
1 Byte 1 to 255
1 Byte 0x81
1 Byte 0xCD: No function
2 Bytes Produced by firmware.
Data
Page: 4-6