1. What is serial communication?
Serial is a very versatile protocol for device communication on a computer (not to be confused with Universal Serial Bus or USB). Most computers contain two serial ports based on RS232.
The serial port is also a common communication protocol for instrumentation equipment; Many GPIB-compatible devices also have RS-232 ports. At the same time, the serial communication protocol can also be used to obtain data from remote acquisition devices.
2. The use of serial communication
Serial communication is done using 3 wires: (1) ground, (2) sending, and (3) receiving. Since serial communication is asynchronous, ports are capable of sending data on one wire and receiving data on the other. Other threads are used for handshakes, but are not required.
The most important parameters for serial communication are baud rate, data bits, stop bits, and parity. For two ports to pass through, these parameters must match:
a, baud rate: This is a parameter that measures the speed of communication. It indicates the number of bits delivered per second. For example, 300 baud means 300 bits are sent per second. When we refer to clock cycles, we are referring to the baud rate, for example, if the protocol requires a baud rate of 4800, then the clock is 4800Hz.
This means that the serial communication has a sampling rate of 4800Hz on the data line. Usually the baud rates for telephone lines are 14400, 28800 and 36600. The baud rate can be much larger than these values, but the baud rate is inversely proportional to the distance. High baud rates are often used for communication between instruments placed in close proximity.
b, Data Bits: This is a parameter that measures the actual data bits in the communication. When a computer sends a packet, the actual data will not be 8 bits, the standard values are 5, 7 and 8 bits. How you set it up depends on the message you want to send.
For example, the standard ASCII code is 0-127 (7 bits). The extended ASCII code is 0-255 (8 bits). If the data uses simple text (standard ASCII code), then 7 bits of data are used for each packet. Each packet refers to a byte and includes start/stop bits, data bits, and parity bits. Since the actual data bits depend on the choice of communication protocol, the term "packet" refers to any communication situation.
c, Stop bit: Used to represent the last bit of a single packet. Typical values are 1, 1.5 and 2 bits. Since the data is timed on the transmission line, and each device has its own clock, it is likely that there will be a small out-of-sync between the two devices in the communication.
So the stop bit not only indicates the end of the transfer, but also provides the opportunity for the computer to correct the clock synchronization. The more bits that are applied to the stop bits, the more tolerant different clock synchronizations will be, but the slower the data transfer rate will be.
d, parity bit: a simple error detection method in serial communication. There are four ways to detect errors: even, odd, high, and low. Of course, it is possible to do without a check digit. For even and odd checks, the serial port will set a check bit (one bit after the data bit) to ensure that the transmitted data has even or odd logical bits.
For example, if the data is 011, then for the even check, the check digit is 0, and the number of bits to ensure the logical high is an even number. If it is an odd check, check bit 1, so that there are 3 logical high bits. The high and low positions are not really checked data, and the simple set logic is high or the logic is low. This allows the receiving device to know the state of a bit and has the opportunity to determine if there is noise interfering with the communication or if the transmitted and received data is out of sync.
3. Commonly used PLC basic interface
a, what is RS-232?
RS-232 (ANSI/EIA-232 standard) is a serial connectivity standard on IBM-PCs and their compatible devices. It can be used for many purposes, such as connecting a mouse, printer, or modem, as well as connecting to industrial instrumentation. For drive and wiring improvements, RS-232 transmission lengths or speeds often exceed standard values.
RS-232 is limited to point-to-point communication between PC serial ports and devices. RS-232 serial communication is up to 50 feet.
b, What is RS-422?
RS-422 (EIA RS-422-A Standard) is a serial connection standard for Apple's Macintosh computers.
RS-422 uses a differential signal, and RS-232 uses a signal that is not balanced with a ground reference. Differential transmission uses two wires to send and receive signals, which is better noise resistant and has a longer transmission distance than RS-232. Better noise immunity and longer transmission distances in industrial environments are a great plus.
c, What is RS-485?
RS-485 (EIA-485 standard) is an improvement over RS-422 in that it increases the number of devices from 10 to 32 and defines the electrical characteristics at the maximum number of devices to ensure sufficient signal voltage.
With the ability to multiple devices, you can use a single RS-422 port to establish a device network. With excellent noise immunity and multi-device capability, RS-485 is the choice for serial connectivity when establishing a distributed network of devices, other data collection controllers, HMIs, or other operations connected to PCs in industrial applications.
RS-485 is a superset of RS-422, so all RS-422 devices can be controlled by RS-485. RS-485 can be serialized with more than 4000 feet of wire.
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