What happen if panelview and sacada write plc5's same address at same time?

Hi,I was required to configure PID loops in pavelview(RIO to plc5) and SCADA system(DH+ to plc5), I am wondering if two system write the same address at same time. What will happen? Can the PLC5 system handle such confict?

The PID values are in the PLC. Any devices can make a change. Last one in, wins.

So,In plc ladder logic, I mean that the hmi and scada must operate the same address in the PLC5. for example, One PID PV Setpoint is Block Transfer Read from hmi (address is N202:0),then I use Mov instruction to change PID control block's SP word. The scada system can only operate the N202:0 address, can not operator directly to PID Control Block's SP word. Is it Right?

Probably, but it depends on your program logic. In any case, if two devices both write to the same address at the same time, there is no problem. Only one at a time will actually write due to addressing, network communications, and rung scanning. As Tom said, the last one that gets in will be the one that gets set in the PLC.

My panelview communicate with plc5 by RIO, Like I said before,
Address N202:0(mov to pid loop's Setpoint) in plc5 is Block transfer Read from panelview by RIO comunication, it is always refreshed by BTR instruction. How can Other SCADA system modify this address's value?
Thanks,

The scada will write to its tag addresses via explicit messaging behind the scenes. This can occur at any time during the PLC scan.
The RIO rack data (real input and output address) transfers will occur between scans, but the BTR/BTW data can also occur an different points during the PLC scan.

So, either device can change the contents of the addresses at any time, and there is really no way to know which will occur first, or last, if they both write to the same addresses.

If you want to ensure that the data remains constant during the PLC scan you should buffer it (make a copy) before the logic that needs a steady value. It appears you are already doing that by copying the N202:0 value into the PID SP. That is a wise thing to do, so your PLC logic has control over when the SP can be changed.

It might also be advisable to use unique addresses for the SCADA and the HMI to simplify troubleshooting and provide a means of tracking where the changes are coming from.

Thanks OkiePC, SO I can change N202:0 from RIO HMI and Scada to ultimately change PID's SP.

How can Other SCADA system modify this address's value?

Click to expand...

Another SCADA system can modify address N202:0 by writing directly to it in the PLC's memory. However it sounds like you may have a situation where the Panelview can write one value, but the SCADA may need to write a different value. Somehow you must resolve which value has priority. The PLC doesn't care, so you must.

I hope both Panelview and Scada can modify N202:0, the last one win and change the value. I think I misunderstand RIO panelview as a real I/O rack in which data are updated within scan period, so other Scada's data modification to N202:0 don't take effect to the logic.Thanks.

As far as your PLC is concerned, it doesn't matter where the value comes from. It could be any device, it could get the value from a thousand HMI screens... last value to hit the PLC will ALWAYS be the one that works.

If you are concerned about limits, or the operators setting values they shouldn't, there are a couple of ways. One, bring the inputs in at separate addresses, then limit them with a LEQ, GEQ, or LIM instruction. Better still, in the PID itself there is a limit you can set, where the value never goes above or below a predetermined number, regardless of the setpoint input to the PID.

Data from your PanelView using BTRs occurs during I/O update scans, as and when the BTR control block is enabled. It is most probable your BTR is either continuous (re-enabled upon completion), or cyclic (timer driven).

The data "write" to N202:0 from your SCADA will most likely only occur if the SCADA tag data value changes.

So having made a change to the setpoint by SCADA, it writes the value to N202:0, then on the next BTR from PanelView N202:0 will get written again.

Summary :- the HMI data will always "win".

Lancie wrote:-

Somehow you must resolve which value has priority. The PLC doesn't care, so you must.

Click to expand...

You will need to set-up the HMI and SCADA to write to 2 different PLC addresses, and then write logic to flag which was the last address to be modified, copying that value to your PID setpoint.

Remember to modify your SCADA and HMI "read-back" to look at the current PID setpoint, not the values in N202:0 and your new address.