| Yes, you can do the two I/O servers with a
switch to change over. You need some cicode to disable/enable the
I/O devices at the same time you switch the switch. To switch the
I/O ports you will need another PLC which both I/O servers can
communicate all the time. That way both I/O servers can check the
state of the switch to disable/enable the states of the I/O
Devices. Also you have to switch the switch if one of the I/O
servers is shutdown. So you need some type of heart beat from the
I/O servers to the controlling PLC.
This is exactly how it was done for one Citect project. They
could only have 1 serial port to each PLC as the PLC were a very
long distance away and they did not want to run another serial
cable. After they completed the project and had written and
debugged the cicode they decided it was a bad engineering solution.
The reasons why, was that it was too complicated and basically a
unreliable solution just to save the cost of installing an extra
cable. The solution is bad as you don't really have full redundancy
and you don't have bump-less transfer. As you can imagine every
time you shutdown one of the I/O servers the serial switch must
change over and the other I/O server must re-establish
communications. This takes time and you get #COM for a short period
of time.
So the answer is you can do this, but don't do it to save money.
You will end up spending a lot of time engineering the software,
commission the system and even then this solution is quite poor.
Only do this if the I/O device can only have 1 serial port.
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