Page 372 - Gas Detection Systems Training Manual

Here is an example of a request to read Modbus holding registers 42104–42106 from slave device 17, sensor 1 (region 1 = 21xx):
Read Holding Register Query
Field Name
Slave Address Function Code Starting Address High Starting Address Low Num Points High Num Points Low CRC Low Byte
CRC High Byte
Example (Hex)
11 03 08 37 00 03 B4 F5
Response
The register data in the response message are packed as two bytes per register, with the binary contents right justified within each byte. For each register, the first byte contains the high order bits, and the second byte contains the low order bits.
Here is an example of a response to the query:
Read Holding Register Response Field Name
Slave Address
Function Code
Byte Count
Data High Byte (Register 42104) Data Low Byte (Register 42104) Data High Byte (Register 42105) Data Low Byte (Register 42105) Data High Byte (Register 42106) Data High Byte (Register 42106) CRC Low Byte
CRC High Byte
Example (Hex)
11 03 06 01 2A 01 8F 03 21 04 5D
The contents of register 42104 are shown as the two byte values of 01 2A hex or 298 decimal. The contents of registers 42105 and 42106 are 01 8F and 03 21 hex, or 399 and 801 decimal, respectively.
SEC 3120 16-bit Modbus Manual Page 32 Of 46 Sensor Electronics Corporation For Version 1.2 and above December 2012 P/N 1580282, Rev. 1, 20121228