Hello!
Please, tell me the meaning of field "More Follows"
And can be number of obj IDs are qual of 0 in answer packet?

 

A single response to the Read Device Identification request may not fit in a single packet, so this function code has the ability to segment the responses using the More Follows and Next Object Id fields. There are two possible scenarios that can occur:

1. Entire response fits into a single packet
In this case, More Follows must be set to 0 and Next Object Id must be set to 0.

2. The response requires more than one packet
In this case the response will have More Follows set to FF and the Next Object ID will be the Object ID of the first object that was not included in this response.

For example:
- Modbus client sends a Read Device Identification request for Basic device identification with Object ID 0.
- Modbus server sends a response that includes Object ID's 0 and 1 (Number of Objects is set to 2). More Follows is set to FF and Next Object ID is set to 2.
- Modbus client sends a Read Device Identification request for Basic device identification with Object ID 2.
- Modbus server sends a response that includes Object ID 2 (Number of Objects is set to 1).

It would not make sense for a response to have Number of Objects set to 0. If an object does not exist on a device, an exception response should be returned instead.

The above is well explained in section 6.21 in the spec here:
https://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf