Explaination
PLCs like AD, TI505, Modicon, Mitsubishi and Omron have linear addressing spaces. This is not a problem until you want to add another device like a motion controller. Since a motion controller needs about 50 to 100 words of memory per axis it is often difficult to add more axes without moving the location of data in the PLC. I have found relocating data to be painful. It is very painful when a HMI is involved. Changing addresses is a pain. One can add the registers for the motion controller at the end of the used memory, but then the axes can't be accessed as a big array of axes. An AB or S7 allow one to just extend an array. The PLC programming software takes care of making room.
The best back planes are ones that are fast and allow one to do things like one can do on a bus like the VME bus. BTW, no PLC back plane comes close to the VME bus even though the VME bus is ancient. In the past the TI505 had the best bus. It allowed intelligent cards like motion controllers to read and write directly to and from PLC memory with no ladder programming. This allowed data to just automajically update in the PLC just as the Control Logix Ethernet/IP allows data on the net to appear as if they are cards on the bus and no BTR or BTW are required. Also, In the past we were often limited to just a few registers in and out. This is the case with the Modicon PLCs. The limited number of registers per card makes communication more difficult because data had to be passed back and forth using some handshaking or a protocol wereas the Control Logix has instant access to 80 status words and 48 command words.
I didn't think Control Logix cards are hot swappable.
An asynchronous protocol is not an advantage. Being fast a flexible is. There is no point in having I/O update faster than the ladder scan. In some cases an input point may be checked serveral times in a scan which may improve response if the input activates a call to a subroutine. In most cases I think this just clutters the program and an interrupt should be used. To make things worse, the Ethernet/IP updates asynchronus to the ladder so there must be some means of keeping the Ethernet from executing commands to the motion controller when the PLC has only partially updated the command field. We have had to add one extra registers to the input and output registers for synchronization.
For motion purposes, I prefer the ladder and the motion controller scans to be synchronous. This means the PLC scan time must be very fast and deterministic. It makes programming easier and the resulting machine action more deterministic.
