Can’t a false value be forced to be set on the DCS via Modbus communication?
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This post was last edited by “Climbing Everest on a Snail” on January 26, 2016, at 18:46. Let me explain in detail. The 485 communication is used to connect to Honeywell PKS; there is a PLC on site, along with a simple interlock system – the pump stops automatically when the liquid level reaches 30%, and it starts automatically when the level reaches 80%. All of these signals are transmitted via 485 to the DCS. What I want to ask is whether it’s possible for the DCS to force a fake liquid level value in order to start the pump ? ? ? Waiting for a reply online.
If this point is hard-wired, that’s fine; or you can go directly to the PLC station and force it
It’s not a wired connection; it’s 485. Is that okay?
If it’s in the QDB connected to the server, there’s no way to force it； If communication with the controller is done via PCDI, then the function block has a simulation function, which acts as a form of coercion.
This post was last edited by Taohualing Jushi on 2016-1-27 08:31. Communication, of course, isn’t an option; every time communication takes place, the new values sent via Modbus overwrite the old values stored in the DCS. A test can be conducted by increasing the communication interval, while monitoring the communication completion bit and a randomly selected communication variable. First, change the variable to any value different from its current value, and then observe that when the rising edge of the communication completion signal appears, the value of the variable will return to its previous value. So, you might be asking, what should I do if I really need to make changes? Easy! Add a Modbus write command to the DCS, with the target address being the one you want to modify. For this specific case, you can modify the DCS logic for starting and stopping the pump, changing it from a single level interlock to one that involves two separate circuits. One is chained, the other is not chained and can be freely controlled
Modsim can be used for simulation
What was said on the 7th floor is correct. In addition to the increase required for DCS, the same increase is needed in PLC as well