Allowing for control from distributed control system & manual failsafes

[fusedev]

Hi Forum land,

Wondering about best practices for programming a PLC to operated by a Distributed Control System interface but also allow for physical panel of switces and failsafes to be used to operate devices in the event of failure of the remote system.

I've not found much because perhaps my inexperience with PLCs prevents me from knowing the right questions to ask the search. Much obliged for any help or directions to look in.

J

[franji1]

Definitely also post on Automation Direct's Community forum - their audience is large and can definitely point you in the right direction(s):

https://community.automationdirect.com

[ATU]

You need to do a risk assessment if this involves operator safety.

[fusedev]

Thank you both: ATU, yes, a risk assessment is a must. Our safety officer is on it. franji1, I've just posted there. Thanks forr the tip!

We have a DL205 PLC controlling a deep vacuum system composed of three pumps.

At the moment, the PLC receives inputs from a panel of switches, which are mapped to X memory

Outputs from the Ladder are mapped to Y memory.

The person who programmed it was new to PLC and Ladder Logic.  I helped once we got to setting up ModbusTCP with the ECOMM100 to allow for control via a distributed control system (EPICS).
The PI would like redundant control. I mapped modbus memory that are copied into the X memory to act as user inputs for the DL205.

At the moment, it is either manual or remote control. The state is changed with a single input bit. Consistency is maintained between control states by using the bits representing one state to update the bits in memory representing the other control state: that is, if we are manually operating and I switch a pump on, the memory is copied to the modbus location. If we are operating remotely, the memory for local is updated.

This is a hack, to be sure and We'd like to create a more robust solution.

The ladder we have uses two system tasks. The copying of inputs occurs at top of scan and copying of outputs at bottom. The direction of copying is toggled by the toggling of a ;RemoteEnable' Bit

How is this sort of thing usually accomplished ? Is it usually accomplished?

Josh