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What is RS232?

Sylvia 2017-05-02 19:36:31
Access control mode of development history RS232 serial communication phase

Twenty years ago, with the computer, computer software, single-chip automatic control technology to flourish, through the computer management of intelligent access control system began to appear. As the computer is usually behind a RS232 communication interface, so access control and most of the need to communicate with the computer and the same equipment used RS232 communication.

RS232 communication parameters and features:
Communication distance: a few meters to tens of meters
Number of networked devices: 1 set

Advantages of RS232 communication:
1 development is simple,
Almost all programming languages ​​have RS232 communication controls and cases.
2 There was no other better way to communicate.
USB, TCP / IP and other commonly used communication methods were either not yet invented, or still in the laboratory test phase.

Disadvantages of RS232 communication:
1 transmission speed is slow, communication distance is short.
Generally stable transmission only a few meters, if extended to ten meters or tens of meters, there will be serious interference and data transmission errors. Communication speed is usually 9600 baud rate (this is a very low communication speed), if you need to download a large number of credit card records and upload a large number of registered card permissions, the user needs to wait for a few minutes or even tens of minutes.
That year the access control software, many can set the communication rate, increase the rate can increase the transmission speed, but will shorten the communication distance, if not shorten the communication distance, it is prone to a lot of interference and garbled. Reduce this rate, you can extend the parameter distance, improve anti-jamming capability, but the communication speed will drop to very slow.
2 can not be more than one device networking.
Can only achieve equipment and computer point-to-point communication, which means that a computer can only access a access controller, a control controller usually can only control a door or two doors or four doors, eight controller Has been very few, not eight controller difficult to do, but the eight-door controller needs at least more than 20 pieces from the card reader, buttons, electric locks the wires, these wires are wound together, resulting in large interference, easy Resulting in access control equipment instability, and access control system is 24 hours without interruption of power supply system, the user's high stability requirements. So, we usually see most access controllers only single door, double door and four door. Can not be connected, it means that a computer usually can only control the four doors, can not meet the needs of a larger user.
At that time, some people think of using multiple serial cards to achieve multiple access control network, the basic idea is to connect the computer more than serial cards, multi-serial card according to the different models were 4, 8, 32, theoretically can be done 255, but this way requires each controller to pull a communication line to the computer, and the length can not exceed tens of meters, the actual operability is very low, so this approach is only short-lived.