First In First Out and Sequencing

Hi all,

I'm new to the PLC world and i'm currently using Schneider Electric Unity Pro XL software. I need advice or sample programming on the problem below.

there are 3 demand and 3 pump (Pump A, B and C). Each demand will start 1 pump and if there is 2nd demand coming in, then the 2nd pump will turn on. If one of the demand is turn off, then the first pump will turn off and then if the 2nd demand is turn off, then the 2nd pump will turn off, following the FIFO rule. Then, if there is new demand coming in, the demand will turn on Pump B instead of Pump A or C.

Anyone have idea on how to write it?

Attach is my program in DFB
http://www.4shared.com/file/SEJKqBjU/test1.html

Please help...

use rising edges of run feedbacks to set internal bits giving you indication which pump was started first, second, and third. This way you will know which pump must start or stop next

counting running time can be a wise idea too

you start the pump with the lowest life runtime and stop the one with the higher actual runtime

thanks for reply,

meaning, taking the rising edge on which pump to start first to determine the next pump to be start..
thanks