Electrician must understand the basic knowledge of the PLC serial communication -

Power operation personnel in the use of PLC controller to be exposed to a lot of communication protocols and communication interface, the most basic communication interface of PLC controller serial communication and the basic do you understand?

1, what is a serial port communication?

serial port is a very general computer equipment communication protocol ( Don't and Universal Serial Bus (USB) Universal Serial Bus or USB confusion) 。 Most computer contains two based on RS232 serial port. A serial port and instrumentation equipment general communication protocol; A lot of GPIB compatible devices with RS - also 232. At the same time, serial port communication protocol can also be used to obtain the remote data acquisition device.

2, and the use of serial port communication

serial communication using 3 thread completed: ( 1) Ground wire, 2) Send, 3) To receive. Because the serial communication is asynchronous, port can be in a line to send data at the same time receiving data on another line. Other line to shake hands, but not a must.

serial communication is the most important parameter is the baud rate, data bits, stop bits and parity. Passage for the two ports, these parameters must be matched:

a, baud rate: this is a measure the parameters of communication speed. It says the number of transmit the bits per second. For example 300 baud said per second sent 300 - bit. When we mentioned the clock cycle, is refers to the baud rate for example, if the agreement we need 4800 baud rate, so the clock is 4800 hz. This means that in serial communication cable on the sampling rate of 4800 hz. Usually baud rate to 14400288 00 and 36600 telephone lines. Baud rate can be far greater than these values, but the baud rate and is inversely proportional to the distance. High baud rate is often used for communication between placed close to the instrument.

b, data bits: this is to measure the parameters of the actual data communication in. When a computer sends a packet, the actual data will not is eight, the standard value is 5, 7 and 8. How to set up depends on what you want to transmit information. For example, the standard ASCII is 0 ~ 127 ( 7) 。 Extended ASCII is 0 ~ 255 ( Eight) 。 If the data using a simple text ( Standard ASCII) Seven, then each packet using the data. Each package is to point to a byte, including start/stop, data bits and the parity bit. Due to the actual data depends on the selection of communication protocol, the term 'package' refers to any communication.

c, stop bit: the last one is used to represent a single package. The typical value is 1, 1. 5 and 2. Because the data is in timing on a transmission line, and each device has its own clock, probably in the communication between two devices appeared a little out of sync. So stop bit is not just said the end of the transmission, and to provide computer clock synchronization correction opportunities. Apply to stop bits digit, the more the greater the tolerance of different clock synchronization, the slower but the data transfer rate and at the same time.

d, parity bit: in a kind of serial communication, simple way of error detection. There are four kinds of error detection method: accidentally, strange, high and low. Of course no parity bit is also possible. In the case of even and odd parity, a serial port will set the parity bit ( A) after the data bits , with a value to ensure that the transmission of data is accidentally a or a logic high. For example, if the data is 011, so for parity, parity bit is 0, the logic high digit is an even number. If it is odd parity, parity bit 1, so you have 3 logic high. High and low is not the real inspection data, setting simple logic high or low logic check. This allows the receiving device to know the status of the a bit, have the opportunity to determine whether there is a noise interferes with the communication or transmission and reception of data are not synchronized.

3, commonly used PLC basic interface

a, what is RS - 232?

RS - 232 ( ANSI/EIA- 232 standard) IBM - On the PC and its compatible computer serial connection standard. Can be used for many purposes, such as the connection of the mouse, printer, or a Modem, at the same time can also meet industrial instrumentation. Used to drive and improvement of the attachment, RS - in actual application 232 transmission length or often faster than standard value.

RS - Is limited to 232 PC serial port and point to point communication between devices. RS - 232 serial communication distance is 50 feet.

b, what is RS - 422?

RS - 422 ( EIA RS- 422 - 一个标准) Is Apple Macintosh computer serial port connection standard.

RS - - 422 using difference signal, RS 232 using non-equilibrium reference signal. Differential transmission using two wires to send and receive signals, contrast RS - 232, it can better anti-noise and further transmission distance. In the industrial environment better antinoise and further transmission distance is a big advantage.

c, and RS - is what 485?

RS - 485 ( EIA- 485 standard) Is RS - 422, because it increases the number of the equipment, increased from 10 to 32, at the same time defines the maximum number of equipment electrical characteristics of cases, to ensure adequate signal voltage. Have the ability to multiple devices, you can use a single RS - 422 set up equipment network. Good ability to resist noise and multiple devices, connect to PC in industrial application of the distributed network equipment and other data collection controller and HMI or other operation, serial connection will choose RS - 485.

RS - Is the RS - 485 422 a superset of, therefore all RS - Equipment can be RS - 422 485 control. RS - 485 can be used for serial passage line more than 4000 feet.

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