1. Advantages and disadvantages of Modbus communication 485/232/422.
( 1 ) RS232 and RS422 are full duplex, RS485 is half duplex.
( 2 ) The difference between RS485 and RS232 is only the physical protocol of communication (namely the interface standard). RS485 is a differential transmission mode, and RS232 is a single-end transmission mode, but there is not much difference in the communication program.
( 3 ) If the PC is already equipped with RS232, it can be used directly. If it uses RS485 communication, just connect a RS232 to RS485 conversion head on the RS232 port, and there is no need to modify the program.
( 4 ) RS-232 only allows one-to-one communication (single-station capability) , RS-485 interface allows connection of up to 128 transceivers on the bus (with multi-station capability)
( 5 ) Similar to RS422, RS485 also adopts a differential signal transmission method, which can reduce the influence of electromagnetic interference and signal attenuation. In addition, RS485 also adopts a multi-master-slave structure, which can support communication between multiple master devices and multiple slave devices.
Second, serial/parallel communication, synchronous/asynchronous communication.
In parallel communication, each bit of data is sent or received at the same time; in serial communication, each bit of data is sent or received bit by bit.
8-bit data is transmitted in parallel, at least 8 data lines and a common line are required, and sometimes control lines such as status and response are required. For long-distance transmission, the price is more expensive and inconvenient, and the advantage is that the transmission speed is fast.
Serial communication only needs one or two data lines. It is more economical for long-distance transmission, but because only one bit can be transmitted at a time, the transmission speed is slow. With the increase of communication signal frequency, the contradiction of slow transmission speed has been eliminated. gradually eased.
Synchronous communication means that the sender and the receiver need to execute at the same time during the communication process, that is, the receiver must wait for the sender to complete an operation before the receiver can start processing data.
Asynchronous communication means that the sender and the receiver do not need to execute at the same time during the communication process. The sender can continue to perform other operations after sending the message without waiting for the receiver to process the data.
3. What is the control of the cylinder machine, and what is the process flow of the cylinder?
The so-called pneumatic actuators are components that use compressed air as power to drive the mechanism for linear, swinging and rotating motions.
Single acting cylinder:
Only one side of the piston is supplied with air, and the air pressure pushes the piston to generate a thrust to extend out, and return by spring or self-weight.
Double acting cylinder:
There is air pressure on both sides of the cylinder piston to realize the forward or backward movement.
4. How to communicate between PLC and HMI?
(1) Modbus communication 485/232/422 connection
（2）ModbusTCP
（3）Ethernet/IP
5. Name the communication methods commonly used in industry
(1) Modbus communication 485/232/422 connection
（2）Ethercat
(3) Profinet and Profibus
（4）Ethernet/IP
（5）Canopen
6. What parameters need to be set for Modbus communication to control the inverter?
(1) Set control source, command source, frequency source
(2) Communication mode selection, RS485/232
(3) Inverter communication address
(4) Communication rate
(5) Verification method
(6) Timeout