GE Profibus Network Scan - How does it work?

[Chris Elston 79]

Does anyone know the details of how a profibus system in a GE system is scanned? I am trying to do some pretty quick stuff over comms, and I am seeing my data arrive around the 30-40 msec range. I was wondering if the profibus master card scans the data in "chunks". Like maybe read and write 1000 registers, then read and write another 1000 registers in PLC scan number 2. So if I had 6000 registers, (which I don't) it would take about 6 PLC scans let's say to update all the profibus registers. Can anyone confirm this is what is going on?

[RussB 22]

I'm no expert here and not knowing your layout I am just speculating. Profibus is a "high speed serial, token ring" network typically with this type the Master will read and write to a slave only when the slave has the 'token' and each slave takes turns with the token. Given this the through-put speed is determined by the amount of data (number of bytes) and the speed of the token. What baud are you at, and can it be increased based on the total length and type of wire? This is all layed out in GFK-2121. This is independant of the PLC scan, however the PLC has a finite window to communicate to these 'smart cards' so it is possible that it is taking multiple scans to see all new data. This was cut-n-pasted from GFK-2121A: Supports all standard data rates (9.6K, 19.2K, 93.75K, 187.5K, 500K, 1.5M, 3M, 6M and 12M Baud) • Master devices determine the data communication on the bus. A master can send messages without an external request when it holds the bus access rights (the token). Masters are also called active stations. • Slave devices include motion controllers, drives, I/O devices, valves, and transducers. Slaves do not have bus access rights and can only acknowledge received messages or send messages to the master when requested to do so. Slave devices are passive stations and require only small portions of the bus protoco

[Peter Nachtwey 15]

No because I have no idea what DP master you are using. What makes a lot of difference, perhaps the most difference, is the bus connection from the profibus dp master to the PLC memory. If the Profibus DP master is built in as in most S7 it is fast otherwise it will be slow because it can only access the Profibus DP master card during the I/O part of the scan. In the case of a S7 345-2 profibus DP master card the Profibus DP comms are slow because the Profibus data must be sent across the S7 back plane which isn't that fast. I have seen a SST DP master update our motion controller 3 times per millisecond when our controller is the only slave device. That is fast. The Profibus DP chips are relatively fast when doing the data exchange and they do it automatically without supervision from a CPU. Where the Profibus DP are lacking is that the current chips were designed for 1980 speed CPUs so they require too many wait states to access.