FBM232 FOXBORO
Large systems with a single machine supporting 300 loops and 65000 I/O points correspond to medium-sized PLCs or above,all using 16-32 bit CPUs.Micro and small PLCs originally used 8-bit CPUs,but according to communication and other requirements,some have also switched to 16-32 bit CPUs.Due to the fact that PLC sales below I/O 64 account for 47%of the total PLC sales,while those between 64 and 256 account for 31%,totaling 78%of the total PLC sales,more research should be conducted on micro and small PLCs.
The hardware of the PLC controller itself adopts a building block structure,and the product structures of various manufacturers are similar.Taking the Japanese Omron C200HE as an example,it has a bus template frame structure.The basic framework(CPU motherboard)is equipped with CPU templates,while other slots are equipped with I/O templates;If there are many I/O templates,the CPU motherboard can be connected to the I/O expansion motherboard through an I/O expansion cable,and I/O templates can be installed on it;Another method is to equip remote I/O slave stations,etc.These all indicate that PLC manufacturers
develop hardware components for users to choose from,and configure them into PLCs of varying scales.Moreover,the openness of this hardware configuration brings a lot of convenience to manufacturers,distributors(agents),system integrators,and end users,and greatly facilitates the sales supply chain.This is a great success experience.
In addition to general DI/DO,AD/DA templates,I/O templates within PLC have also developed a series of special functional I/O templates,which have opened up a way for PLC to be used in various industries,such as ASCII/BASIC
emplates for barcode recognition,PID templates for feedback control,high-speed counting templates for operation control and mechanical processing,single axis position control templates,dual axis position control templates,cam locator templates,radio frequency identification interface templates,etc,This will have great development in the future.In addition,related components for input and output,as well as input and output electrical isolation and ground isolation in strong interference situations,will also be improved.
The CPU and memory in the PLC cooperate to complete control functions.It is different from the DCS system that processes parameters such as temperature,pressure,and flow rate.It adopts a fast scanning cycle,generally ranging from 0.1 to 0.2 seconds,while faster ones use a scanning cycle of 50ms or less.It is a digital sampling control system.
In order to complete control strategies and replace relays,a set of control algorithm functional blocks(or subroutines),called instruction systems,have been developed for users to complete ladder diagrams of control systems similar to relay circuits.These blocks are fixed in the memory ROM and can be called by users when programming applications.Instruction systems can be roughly divided into two categories,namely basic instructions and extended instructions.The instruction system of general PLC is divided into basic instructions,timer/counter instructions,shift instructions,transfer instructions,comparison instructions,conversion instructions,BCD operation instructions,binary operation instructions,increment/decrement instructions,logical operation instructions,special operation instructions,etc.These instructions are mostly similar to assembly language.
