The full name of RS232 interface is "Technical Standard for Serial Binary Data Exchange Interface Between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)". The standard stipulates that a 25-pin DB25 connector is used, the signal content of each pin of the connector is specified, and the level of various signals is specified. The DB25 serial port generally only uses three pins 2 (RXD), 3 (TXD), and 7 (GND).
With the continuous improvement of the equipment, the DB25 pin is rarely seen now, and the DB9 pin is used instead. For the interface, the pins used by DB9 are different from those of DB25. They are 2 (RXD), 3 (TXD), and 5 (GND). Therefore, the RS232 interface is now called DB9.
Since the RS232 interface standard appeared earlier, it is inevitable that there are deficiencies, mainly as follows:
(1)it is incompatible with the TTL level, a level conversion circuit is needed to connect with the TTL circuit.
(2) The transmission rate is low. In asynchronous transmission, the baud rate is 20Kbps.
(3) The interface uses a signal line and a signal return line to form a common ground transmission form, which is prone to common mode interference, so the anti-noise interference is weak.
(4) The transmission distance is limited. The standard value of the maximum transmission distance is 50 feet. In fact, it can only be used at about 50 meters.
RS485 has several advantages over RS232:
(1) The electrical characteristics of RS-485: Logic "1" is represented by the voltage difference between the two wires as +(2-6) V; Logic "0" is represented by the voltage difference between the two wires as -(2-6) V Said. The interface signal level is lower than RS-232, it is not easy to damage the chip of the interface circuit, and the level is compatible with TTL level, which can be easily connected with TTL circuit.
(2) The highest data transmission rate of RS-485 is 10Mbps.
(3) The RS-485 interface is a combination of balanced drivers and differential receivers, with enhanced anti-common-mode interference capability, that is, good anti-noise interference.
(4) The standard value of the maximum transmission distance of the RS-485 interface is 4000 feet, which can actually reach 3000 meters, and the RS-232 interface only allows one transceiver to be connected on the bus, that is, single station capability. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has multi-station capability, so that users can use a single RS-485 interface to easily establish a device network.
The communication on av over ip has RS232, RS485, IO/IR, relay
Because the half-duplex network composed of RS485 interface generally only needs two wires (we generally call it AB line), the RS485 interface adopts shielded twisted pair transmission.
