Feature abort polling on timeout

[wilbersl]

Problem:

Like here I had some issues with polling multiple devices when one or more don't answer. Without configuring timeout and retries the Modbus clients default to 3 seconds and 3 retries, so each request to a non responsive Modbus Slave/Server takes 9 seconds, before raising a ModbusIOException.

Solution:

Allow for configuration of timeout and retries. I also implemented a service wide configuration skipontimeout that, when set true, skips polling other chunks from the device when a ModbusIOException is raised.

Other possible solutions:

• Multithreading or setting up multiple services for polling different clients would work with Modbus TCP
• Multiple RS485 controllers to separate Modbus RTU connections