flags for plc communication

  • Thread starter iman mulyaman ginanjar
  • Start date

Thread Starter

iman mulyaman ginanjar

Dear All Members,

I've some questions about using flags for PLCs communication. How does the flag work in PLCs communication, what is its function? And, how is the application of this in ladder diagram?

Thanks a lot!
Iman M.G.


Eduardo Manuel C. Cipriano

Dear Mr. Iman

I understand that you are trying to communicate different PLC eventhough you mention using RS232 Bus you did not mention the type of Protocol is it Half Duplex or Full Duplex, anyway the most important thing is to know if both of your PLCs have the same Protocol to be able to make them talk to each other, most of the PLC that i've used have different communication configurations
techniques so its hard to explain which and what to do during configurations, some of them uses an internal configuration menu, while the other uses other software packages, some are using ladder like in allen bradley RIO protocol they uses Block Transfer in their ladder and you have to configure some dip switches.

While on the functions of Flags in the PLC they are usually memory registers/bits used by the PLC cpu and also can be used by the user in their
program one particular example is the 1st scan bit which is usually used to reset any data registers during power up condition of the PLC, while ordinary registers are local registers that can be used to store data values which in turn can be transmitted to the other PLC link in your network, both registers can be used and it takes on the design and intention of the programmer on what he intends to achieve

On loading programs once you have a network in the system you can download any program from any remote computer that is a member of that particular network like in AB they have this DH+ network and a server called the RSlinx which is
used for dowloading in the network.

best regards............

Eduardo Manuel C. Cipriano
Sr. Systems Engr.
Yokogawa Phils. Inc.