HMI communication problem with S7-300 (when scan time is high)

Hi Experts,

Here is the problem, a HMI is communicating with a S7-300 system over an ethernet connection (using ISO TCP/IP Protocol). The scan time in the PLC is rather high (50 - 70 ms). The HMI updates slow when the scan time is high but if we download a simple PLC-application (with a low scan time) then the communication/update is OK.

I assume that the problem is caused due to that the PLC only answers to HMI's requests in the end of the scan cycle. I'm more familiar with Mitsubishi PLC-systems, and Mitsubishi Q has a function called COM_M that you can add in the ladder, this function will make the PLC answer requests directly (and not in the end of the scan cycle).

So, the question is, are there any settings/functions in the S7-300 that will improve how the PLC handle requests from the HMI? All ideas and suggestions are most appreciated.

Thanks,

//Supporter

Which CPU are you using ?

Hi,

It's a S7-317 PN/DP.

//Supporter

I am not sure, that will help ...

In WinCC (if you use WinCC): System Parameter of connection -> Cycle Management -> Uncheck "by PLC"

HI


Some thing you can do :
In the PLC under hardware - CPU properties - Cycle/clock memory ... Scan cycle load from communications you can adjust the percentage spend on comms BUT read the help file !!! because it can have strange results if the values are changed to much.

You don't say what HMI youre using but keep your tags or variables grouped in one block , where possible scan the status only when required.

Maybe you should try and find out what is causing the 'huge' cycle time .Are you using profibus DP .Cause we found out that if you got a lot of ET200 IQ it increases the cycle time.You could at a profibus comms module and connect the ET200 to this module and unload you CPU.

Greetings
Eric

A 317 and scan times of 50 - 70 ms - that's some big program! As aikona says you can adjust the percentage of scan time allotted to communications, but if I remember correctly the default is 20% and that ought to be enough normally. Which HMI are you using, and are you doing a lot of logging?

keep your tags or variables grouped in one block

Click to expand...

This is really the killer, if your I/O is scattered all over the place, especially digital I/O (Bits) you can waste a heck of a lot of transmission time. Data from the PLC to the HMI is usually transferred WORD- or DWORD-wise. If your data is scattered it may be worth mapping all the HMI data to a contiguous area of memory or keeping it all in an HMI-DB so as to optimise data-transfer.