Hi all,

TIA Portal V15.1, TP900 Comfort Panel, 1515F PLC.

We are trying to set an INT PLC tag from the HMI, through an intermediate mapping DB. The mapping DB is mapped to/from the “actual” DB by two MOVE instructions, one called before the FB and one called after FB. To be sure, I placed the MOVE commands in separate program cycle OBs called before and after the rest of the code respectively.

The problem is: about 80% of button clicks in the HMI are lost. They are somehow overwritten by the second MOVE command before the value makes it into the “actual” DB and FB. About 20% of the time the set value is captured correctly.

When the second MOVE command is disabled it works fine (but then the PLC wouldn’t know about the value).

(The reason for the mapping DB is because the Comfort HMI cannot just look at individual DBs when multiplexing the data for popups, it can only point to an array where each element is of the UDT type. So, our “actual” DB is of type UDT, and the mapping DB is Array[0..n] of type UDT. We are mapping one element of the array to/from the “actual” DB.)

It looks like what’s occurring is that the HMI writes are asynchronous, so 80% of the time the first MOVE is already done by the time the HMI writes the value into the mapping DB. Then on the second MOVE, the value from the HMI then gets overwritten by the “old” value from the “actual” DB.

We may need to use separate “read” and “write” HMI interface DBs, multiplying our data structures still further…

However, can we make the HMI-PLC driver work synchronously instead? What I mean by this is for any values set by the HMI to wait until the beginning of the next PLC scan cycle to be set, rather than jumping in halfway through a scan.

Angus

This is exactly what is happening: https://support.industry.siemens.com/cs/ww/en/view/109478253 

The recommended solution is to use two DBs. One for reading and one for writing.

Can creating two DBs be avoided? Is there an option for the HMI to write synchronously?

Regards,

Angus

I have been unable to find a way to set the HMI to write synchronously. Work-around as follows:

The cause of the problem was the asynchronous writing to the PLC from the HMI. The write from the HMI was falling after the initial MOVE command, too late in the scan cycle to be captured. The value in the mapping DB was then being overwritten with old data by the second MOVE command later in the scan cycle.

Our solution was to place all commands from the HMI into a specific structure within the UDT. At step 5 (diagram attached above), we now move each sub-structure of the UDT separately, except for the command structure which is not copied back to the mapping DB at all. This is now tested and working reliably without any problems – each command from the HMI is captured in the PLC.

If the HMI commands are spread throughout the UDT, an alternative is to create two mapping DBs, one for write and one for read. We decided against this as it would have created even more DBs, required reworking the HMI screens and our commands were already mostly within one structure anyway.

Angus

Hi,

can You describe your solution more precisely, i can't catch the point.

Thanks