We have several assy lines that use RFID systems for tracking.  We use Escort Memory Systems www.ems-rfid.com with plc 5's and SLC's.  We also use an Omron RFID system talking to some SLC's on one assy line but they are slow with communications.  We have had excelent luck with the EMS.  Need more info on what your system is and how you plan to use it.

We plan on using it as a timing device to see how long log trucks are in the mill (ex. when they come in, when they get to the unloading area and when they leave) then have this info to be fed into the SLC's

We have use a couple of differant methods to communicate to the RFID reader heads with SLC's.  One system uses 1746-BAS cards and another uses a special module specifically designed to fit in the SLC's backplane.
As with both types of cards they use the M0 and M1 files to transfer information to and from the PLC processor.
Both of our PLC5 systems also use basic cards for the communication to the reader heads.
There are 2 basic types of systems that I can tell.  One type stores information in the tag besides the tag number.  As you can read and write to the tag.
With the other type you can only read the tag number and you must handle all the tracking information in the PLC.
Either type is good.  We use both.
Have you picked a specific type/brand/model of RFID?

We're probaly gonna go with the HYPER X system from BALOGH and then send the info to a SLC500 Basic Module, and then send it to the main PLC via radio modem.  As far as the tags go we havent decided whether read only or read-write, right now we're planning on about 30 tags, which would you recommend.  We would like to use preferably one reader at the entrance/exit and the two others, one before the loading site and one after the loading site in order to keep track of how long the truck was waitng, being loaded , etc.

Micky
mickyfranks@xspedius.net

Of the two systems personally I prefer the read only.  All the tracking information is kept in the plc registers that you can view at any time.  It is pretty basic you just read the tag number, and use indirect addressing to access the appropriate registers.