Slow Communication between DCS and PLC

Hi Experts,
I have a Siemens PLC inetrfacing with Foxboro DCS. The communication is very slow, takes around 12-15 sec for a DO from DCS to PLC but for other IO types it's ok, I mean for DI to DCS it takes 1-2 sec and same for the AI from PLC to DCS. Any guess what could be the reason. I don't doubt the interface since it's working normal for DI and AI and I have also made sure that there is no duplicaton of address in either of the Systems.Please help.

If you have hardwired handshakes e.g. DO in the PLC wired to DI in the DCS it should happen in one scan. Are you sure it's not over a serial link?
Communication over a serial link can be slow but usually possible to speed it up by re-arranging the order, If you are reading words from many different parts of memory each one requires a different message whereas if the words are compacted together it goes much faster.
Roy

Hi Matson.
The communication is serial using modbus. The diffrent loacation of the DO doesn't seem to be the case becoz I'm forcing directly at the Packout block.

As I said before, it sounds as though your communication link is very busy anf the DO task is given a low priority.
I'm sorry I am not very familiar with the two pieces of hardware hopefully someone else will chip in.
If you think about it a discrete bit (part of a word) should be just as fast as an analog word (16 bits). Perhaps try adding one word of discrete bits next to an analog word that you know is updating fast.
Good Luck
Roy