Data Exchanges Between PLCs

Hello all,

I'm quite new to PLCs domain, and I wonder if opportunities of data exchanges between PLCs would suit my needs.

We work on a future architecture where we should have 3 PLCs, one safe and two standard (probably redundant). I was wondering:

- what types of data I could share between them?
- what amount?
- Is there any limitation if those PLCs are not of the same manufacturer?
- Finally, what is the transfer time between PLCs : can it be compatible with control loops?

Many thanks in advance,
Points you raise seem hypothetical:

1. Turn your point back to you - what type of data exchanges would you want.

2. When you refer to 3 PLCs, are you looking for dual redundant/2oo3 (2 out of 3) voting? Both are common for high reliability systems: what are trying to achieve?

3. You can send any data you can access, but what is the justification to 'share' data?

4. There is no limitation of the amount of data you can send, except the tradeoff of the more you send, the longer it takes to send/receive.

5. Most PLC manufacturers have different protocols so are generally incompatible. The workaround here is to use 'Modbus' which is an internationally used protocol. Search this site to see it's usage.

6. Transfer time can be slow or fast to suit. Modbus using a serial link with short data strings at low baud rate may take up to 1000msec - using a fibre optic link it is a case of "how fast do you want it"?

Siemens S7-400 dual redundant with dual fibre connection configuration gives you hot standby where counter/timer values and data table areas are copied across every scan. The tables can be fairly large and scan times can be as low as 10-15ms, and operates transparently to the user.

7. So yes, can be used in control loops.