Issue
How can a 16-bit signed integer value be correctly interpreted and displayed when read from a Modbus register?
Product Line
Satchwell BAS & SigmaEnvironment
Sigma IC-Modbus
Cause
Modbus registers store data in 16-bit format. When dealing with signed integers, the value range is from -32,768 to +32,767. If a value appears as 65xxx, it typically indicates a negative number due to the way signed integers are represented in binary (two's complement format).
Resolution
To correctly interpret a 16-bit signed integer from a Modbus register, use a lookup table that maps the raw register values to their corresponding signed integer values.
Here’s a simplified version of the mapping:
| Raw Value | Signed Integer |
|---|---|
| 0 | 0 |
| 32767 | 32767 |
| 32768 | -32768 |
| 65535 | -0 |
Explanation:
- Values from 0 to 32767 represent positive integers.
- Values from 32768 to 65535 represent negative integers, using two's complement encoding.
