adding a 2nd HMI

[Skidood]

Hi Folks.
I am gearing up to add a 2nd identical HMI to a project. This 2nd HMI will basically be an exact clone of the 1st one. The equipment we are controlling will be operated from either one HMI or the other, depending on the needs of the customer on that particular day. It will not be possible or allowed for 2 different people trying to operate the unit from both HMI's simultaneously. The HMI is told what screen to display by monitoring an integer value in the PLC. Both HMIs will display the same screen at any given moment in time.

My concern is, there are some pushbuttons (momentary and toggles) on some screens that trigger various actions in the PLC. So if one button is pushed while the same button on the other HMI is obviously NOT pushed, what will happen? Is the other HMI forcing that bit OFF hence creating some kind of conflict for that bit in the PLC?
I'm using a couple of Automation Direct EA9's and a MicroLogix 1400...one HMI is connected to the PLC via ethernet cable and the other is connected thru a RS-232 cable
If the bit in the PLC (which is controlled by the pushbutton) changes state as it should, even for one scan, I should be OK as in most cases I am latching something but I was wondering if anyone has any advice or input on this. TY

 

[Steve Bailey]

So if one button is pushed while the same button on the other HMI is obviously NOT pushed, what will happen? Is the other HMI forcing that bit OFF hence creating some kind of conflict for that bit in the PLC?

You don't have to worry about that. For a momentary pushbutton object the HMI writes a value of 1 to the bit in the PLC when you touch the button on the HMI screen. It only writes that value once. Then, when you release the button on the HMI screen,. it writes a value of zero to the PLC bit. It only writes that value once.

 

[robertmee]

As Steve said, the comm server operates on change in state and writes the value once. The only concerns about cloned HMIs is whether you have any issues with one being logged in at some security and the other isnt, or whether you have any alarm ack handshaking where one HMI would clear alarms on the other HMI.

 

[chavak]

When we had multiple HMI, what we normally do is, add in some sort of 'Take Control' concept. It can be either external selector switch or HMI buttons. And in the HMI all the manual controls are grouped in a series of pages, where the access to it is only granted for whichever HMI is in control. So manual controls are only rendered to the HMI who is in control. This is to prevent any safety mishaps. [Accidental trigger of the actuator by someone else from the other end of the machine]
Only caveat is after the usage, the operator must release the control, so later on who ever can access the HMI from either location, can take over the controls. Normally we use selector switch with illumination, illuminated when in control
When the machine is newly delivered, operators may forget to release it, but after a couple of days it works as intended.

Regards