RS232 and RS485 features

RS232 features

 　　RS-232 is one of the mainstream serial communication interfaces. Since the RS232 interface standard appeared earlier, it is inevitable that there are deficiencies, mainly in the following four points:

 　　(1) The signal level value of the interface is high, which is easy to damage the chip of the interface circuit. The voltage of any signal line of the RS232 interface is in a negative logic relationship. Namely: logic "1" is -3-15V; logic "0": +3-+15V, noise tolerance is 2V. That is, the receiver is required to recognize signals higher than +3V as logic "0", signals lower than -3V as logic "1", TTL level of 5V is logic positive, and 0 is logic negative. It is not compatible with TTL level, so it needs to use level conversion circuit to connect with TTL circuit.

　　(2) The transmission rate is low. In asynchronous transmission, the bit rate is 20Kbps; therefore, in the 51CPLD development board, the integrated program baud rate can only be 19200, which is also the reason.

 　　(3) The interface uses a signal line and a signal return line to form a common ground transmission form. This common ground transmission is prone to common mode interference, so the anti-noise interference is weak.

 　　(4) The transmission distance is limited. The maximum transmission distance standard value is 50 feet, but it can only be used at about 15 meters.

　　RS485 features

　　 When the communication distance is tens of meters to thousands of meters, the RS-485 serial bus standard is widely used. RS-485 uses balanced transmission and differential reception, so it has the ability to suppress common mode interference. The standard value of the maximum transmission distance of the RS-485 interface is 4000 feet (approximately 1219 meters), which is actually up to 3000 meters. In addition, the RS-232-C interface allows only one transceiver to be connected on the bus, that is, single station capability. The RS-485 interface allows the connection of up to 128 transceivers on the bus, that is, multi-station capability, so that users can use a single RS-485 interface to easily establish a device network.