Sharing Tags in RSView...bad idea?

Usually my projects with multiple HMI's have the HMI's displaying different controls for the section of the machine it's mounted to. I have to program two that are mirror images of each other. I was wondering if each HMI should have their own tags or if it was ok to use the same tags? It wouldn't be hard to make new tags for them all, I guess it would be nice to know what other people are doing.

Thanks,
Jay

Short answer: It shouldn't be a problem.

Long answer: If you're having doubts, there may be a good reason. Have you thought through how the HMIs will behave if they're sharing tags?

Here's an example: Are you one of those types who programs button status based on the PLC? What will happen on HMI #2 if someone pushes a button on HMI #1?

Another example: Will either HMI be controlling the system tags of the PLC (date/time) or will the PLC be controlling the page number on the HMI?

Good answer, akreel. I like your sig too. For a simple app you're probably OK. You'll have to think about it/test for a more complex app. In most cases I would consider typical behavior in akreel's first example to be correct.

Having multiple clients share tags really doesn't create subtle problems unless you're doing weird stuff.

You get into sticky issues if you're making the HMI do things that the PLC probably should be doing (usually with script). The typical thing that will get you in trouble is writes to the PLC.

1. Watch out for possible race conditions. Things like the HMI conditionally incrementing/decrementing a tag might cause this.
2. Be aware of possible lag between writes on one HMI and read through on another.
3. Keep operations as "autonomous" as possible, think transactions, - if you have a multi-step process, you usually want the operation to occur at the same time - not as the user inputs data.

akreel said:

Short answer: It shouldn't be a problem.

Long answer: If you're having doubts, there may be a good reason. Have you thought through how the HMIs will behave if they're sharing tags?

Here's an example: Are you one of those types who programs button status based on the PLC? What will happen on HMI #2 if someone pushes a button on HMI #1?

Another example: Will either HMI be controlling the system tags of the PLC (date/time) or will the PLC be controlling the page number on the HMI?

Click to expand...

If you want them to use the same tags, I'd set up the programming so only one can be used at any given time. If one screen is being used and someone tries to use the other, give them a message stating that another screen has control.
Even if you do that, I'd still use different tags for each HMI.

Is this for View32? If so, while you can clone the tag database, try to only use things that are related to the machine section with the HMI. Ditto if this is ME.

For Factory Talk View, SE, this is all a moot point.

That seem archaic. There are some applications that require this level of certainty in their execution - for those I wouldn't try to mold a single user application into a multiuser environment. If you constrain yourself like that, in a general sense, you might as well mount hard wired push buttons to a panel and call it an HMI.

kbpatton said:

If you want them to use the same tags, I'd set up the programming so only one can be used at any given time. If one screen is being used and someone tries to use the other, give them a message stating that another screen has control.
Even if you do that, I'd still use different tags for each HMI.

Click to expand...