ABB ATS Communication & IF statements vs CASE

Hi all,

I am facing issues with communication to the TruOne ATS of ABB via plc. Plc is M251 of SE. Majorly 2 issues:

1- writing to the modbus register via plc is not not reliable. Like, we can write the values once or twice, but after couple of instances, the ATS doesn’t action on the command we send. Surprisingly, it does read them. The reason why I m sure it reads them is that on the parameters section, it shows those settings. For example, if I want the preferred source to change from S1 to S2, it will be shown on the ATS screen, but it won’t just switch after switching a couple of times. But if I send commands via modpoll, it is smooth and switches as many times as we ask it to. But once it goes in that glitchy mode of not responding (after we send commands via plc), it won’t even respond to the commands from modpoll.

2- case statements vs IF statements.
For the same program, i tried writing values to thr register using if condition, and even though the logic was fine, values were just not being written (I could see the commands being sent, but not getting written in to the ATS register at all). But when I tried same with case statements, it worked.

Any help will be greatly appreciated.

How fast do you write the Modbus register?
Use triggered writes for example when the value changes.

IF and CASE is the same thing.
You are doing something wrong.

Thanks Kal. But why is it then working fine with modpoll? And if statements are not writing anything, while case statements do. I will review thoroughly the code today again surely

You are probably using multiple IF statements instead of ELSIF.
In such case (hah!) the last IF statement will be triggered in the same scan cycle regardless of the other statements being true.

CommissioningMan said:

You are probably using multiple IF statements instead of ELSIF.
In such case (hah!) the last IF statement will be triggered in the same scan cycle regardless of the other statements being true.

Click to expand...

I guess that is correct, but the main issue was spotted in the code that was writing the values to the registers. All good now!

CommissioningMan said:

You are probably using multiple IF statements instead of ELSIF.
In such case (hah!) the last IF statement will be triggered in the same scan cycle regardless of the other statements being true.

Click to expand...

I dont understand the reason though...why will the last IF be true is conditions are not met? You mean all the IF statements can be triggered in the same cycle if conditions are met?