How manufacturers should document scaling, signed values, and exceptions

Good Modbus documentation must tell the user how to turn raw data into the correct value. Three areas are especially important: scaling, signed values, and exceptions.

Scaling

Many devices do not store values exactly as shown to the user. Instead, they store a raw number that must be scaled.

Example:

• raw value 253
• actual temperature 25.3 °C
• divide by 10

The manual should say this clearly. Do not expect the user to guess.

For each scaled value, document:

• the raw format
• the scaling rule
• the final unit
• one example

Signed values

Some values can go below zero. If that is possible, the manual must say whether the register is signed.

Without this, a negative value may be read as a large positive number.

For each signed value, document:

• signed or unsigned
• value range
• one example with a negative value if relevant

Exceptions

Not every request will succeed. The user needs to know how the device behaves when something is wrong.

Document:

• unsupported function codes
• invalid register requests
• write attempts to read-only registers
• Modbus exception codes if used
• special fault values if any are returned

Why this matters

If scaling is missing, values look wrong.
If signed format is missing, negative values look broken.
If exception behavior is missing, support work increases because users do not know whether the problem is their request or the device.

Final note

A good manual should tell the user exactly how to read the value, how to interpret it, and what happens when something goes wrong.

Consulting

Manufacturers can hire me for help with Modbus device documentation, testing, and troubleshooting.