Need help with Digi Connect ME4 & Digi One IA Modbus Timeout Settings

Question

We have imbedded the Digi Connect ME4 in a couple of our products and also use the Digi One IA in many of our systems.
I am trying to fine tune the various IA Modbus/RTU Serial Slave protocol timeout settings to reliable generic values.
Most of my calculations are based on 90000649_a.pdf, Understanding Timeout Settings in Digi One IAP.
The default values in the Connect ME4 are:
ia_master char_timeout=30000 message_timeout=2500
ia_serial char_timeout=20 slave_timeout=1000
Why are there two char_timeout parameters and why is the default value for the master so large?
There are many other parameters in the Connect ME4 configuration that are not exposed in the web interface, such as:
ia_master slave_timeout=1000
ia_serial message_timeout=2500
Are these parameters unused?

Answer 1

There are two 'char timeouts', because they are really 'inter-byte during receive action' timeouts. So your master task is receiving via Ethernet - so the time between two TCP segments is hoped to be less than 30,000msec, but over WAN/cellular can be large. Generally, 1 MB/TCP msg = one segment (aka: packet), but some hosts do things like write first the 6-byte header, then the core message, which results in 2 TCP packets being created (& 2 ACK, etc).

Your slave task is receiving on serial, so the gap is more as you'd expect, thus the short 20msec gap.

The other two settings are used for a serial master talking to a network slave. So they are 'unused' in your context.