Hot Standby TMD's

[Petty_boy]

Hi,

I'm looking to install 2 TMD's in a hot standby configuration.   Both TMD's will run in parallel with each other.
An HMI will be attached to each PLC RS232 comm's port but only reading / writing to one via modbus etc

Each PLC will monitor the other PLC's watchdog output (is there one?) if the watchdog contact is active the comm's will be switched over via output relay or similar to the second PLC and vice versa.

This leaves me with a problem, the data in the first duty PLC will need to be constantly synchronised with the standby PLC so when the PLC's change over any timer values,output commands written from the HMI etc are the same.

I will probably need to use at least two extra TMD's as remote I/O in the field collecting data, reading inputs etc.  this will also need to "switch" onto the standby PLC......

Is this possible? has it been done before??

[support]

You may be able to construct a periodic timer or simply use a clock pulse to provide a heart beat output from one PLC to another. The heart beat will reset a count down timer in the PLC that is monitoring the other PLC. If the heart beat stops then the timer times out and the PLC knows that the other PLC is not actively working and it can control a switch to switch over the comm port that is attached to some relays.

The PLC that owns the COMM port from the HMI would act as a master and constantly check the data that was written from the HMI. If the data has changed the master PLC will write to the slave PLC via RS485 port so that the data is updated to the slave PLC. This way the two PLC will have synchronized data. If the current master PLC stop working, the slave PLC would detect the condition and switch over the COMM to itself and it now assumes the role of the master and the original master will become slave once it start functioning again.

There are some complexity to achieve what you need but it is possible with some careful planning. The relays that switch the RS232 port from the HMI to each PLC would be a bit complicated to design. You will have to plan out how the two PLCs can determine which is the master and how to switch the COMM port to itself.

The additional T100MD+ PLCs to be used as remote I/O may be connected to the same RS485 bus that the two PLCs are using to communicate. Of course again only the PLC that is the current master would initiate the host link commands using the NETCMD$ command or READMODBUS/WRITEMODBUS command to talk to the remote I/Os.