s7-300 profibus and eurotherm 2500 temp controller

[defender]

This isn't a request, just a sort of pointer to the solution in case anyone else encounters the same problem as me. When writing PID values to the 2500 via profibus, control goes completely wrong. Unplugging profibus cable makes control work fine. Bit of a bugger to solve as you cannot communicate with the 2500 via serial and profibus at the same time... Basically the eurotherm controller restarts the PID loop each time it gets a new value, this means every profibus scan. There are 2 solutions. 1. Easy and quick - write the pid values for about 1 second to the device and then write 32767 (#7fff) constantly to the controller, this value is out of range so is ignored and the controller functions as normal. 2. Longer winded, but better. Create a new GSD file and tick on demand. Use on demand mode to write directly to the tags only whenever you change the data. This is explained in a manual that does not come with the 2500, but is available on the net. I found it tonight, of course after I had spent a day working it out for myself (no net in this plant). Go here for on demand explanation. Apart from this problem my experience with the 2500 is very good. Much cheaper than a bunch of normal controllers but without the hassle of programming your own PID loops, and it comes with good old autotune.