Modbus devices with limited requests

Why can't I read data from my Modbus device?

Some Modbus slaves can only handle a limited number of registers per request. If Evolo optimizes the query and accesses an invalid register range, the entire query will fail.

How can I fix this?

• Reduce the maximum query size: Set the maximum to 1 or 2 (depending on 16-bit/32-bit). This prevents the query from including invalid registers.

• Increase the delay between queries: This gives the slave more time to respond.

Example of an error situation:

• Valid registers: 1, 2, 3, 4

• Invalid registers: 5, 6, 7

• Evolo It might try “address 1, count 7,” fail due to 5–7, and also throw an error for 1–4.

The error appears in the system log as:

evolo-gateway [error] 2025-05-20 14:10:44: Modbus exception 2: Illegal data address… slave: 1, register: 0, count: 78

Where can I find the settings?

• Maximum query size and delay: under Advanced Settings in the gateway setup.
  

  

Where can I find information about the maximum query size?
Check the Modbus device manual or contact the manufacturer/supplier.