RS232, RS485, RS422 interface as shown in FIG,
RS232 disadvantages:
High (1) signal level value of the interface, the interface circuit chip is easily damaged, and because the level 232 is not compatible with TTL level it is required in order to use the level conversion circuit is connected to the TTL circuit;
(2) a low transmission rate in asynchronous transmission, the baud rate is 20Kbps. Now thanks to new UART chip, baud rate 115200bps (1.832M / 16);
(3) an interface using a signal line and a return line to form a transmission signal in the form of a common ground, such co-prone transmission of common mode interference, so that noise immunity is weak;
(4) a limited transmission distance, the maximum transmission distance of 50 m standard value, in fact, can only be used at about 15 m;
For lack of RS232, so there have been some new interface standard, RS485 is one of them, has the following advantages:
(1) The RS-485 Electrical characteristics: logical "1" to the voltage difference between the two lines represents a + 2V ~ + 6V, a logic "0" to the voltage difference between the two lines represents -6V ~ -2V. Interface signal level lower than that of the RS-232-C, the interface circuit chip is not easily damaged, and the level compatible with TTL level, TTL circuits connected easily engraved.
(2) the maximum data transfer rate: 10Mbps
(3) a combination of a balanced RS-485 interface driver and the differential receiver, resistance to common mode interference capability, i.e. a good noise immunity.
(4) Maximum transmission distance RS-485 interface standard value of 4000 feet, in fact, up to 3000 meters.
(5) RS-232-C interface connected to the bus allowing only one transceiver, i.e. a single station capability; and the RS-485 interface to the bus only allow up to 128 transceivers, i.e., having the ability to multi-station, so that the user you can utilize a single RS-485 interface to easily establish a network device.
RS-422 and RS-485 Contrast:
Electrical performance RS-485 RS-422 with exactly the same. The main difference is that:
1, RS-422 has four signal lines: two transmitting (Y, Z), two receiver (A, B). Since the yield of RS-422 is separate from the hair can be sent and received simultaneously (full-duplex).
2, RS-485 only two data lines: transmitting and receiving both A and B. Since the yield of RS-485 is made common with the two lines, and so can not be made simultaneously receiving (half-duplex).
In summary, the difference between RS232, RS485, RS422 as follows,
| interface | Way of working | Logical properties | Transmission rate | Transmission distance | Immunity | From the number of stations | Number of signal lines |
| RS232 | Simplex | A logic "1": - 5 ~ -15V A logic "0": + 5 ~ + 15V |
In asynchronous transmission, the baud rate is 20Kbps. | Standard maximum transmission distance is 50 meters, in fact, can only be used at about 15 meters. | A common-mode interference, noise immunity is weak. | 1 | 9 |
| RS485 | Half-duplex | A logic "1": + 2 ~ + 6V A logic "0": -2 ~ -6V |
Maximum data transfer rate of 10Mbps. | The maximum transmission distance is 1200 m standard (9600bps), the fact up to 3000 meters. | Differential transmission, strong anti-interference. | 128 | 2 |
| RS422 | Full-duplex | A logic "1": + 2 ~ + 6V A logic "0": -2 ~ -6V |
Maximum data transfer rate of 10Mbps. | The maximum transmission distance of 4000 feet (1219 m) | Differential transmission, strong anti-interference. | 128 | 4 |
Note: 1, referring to the same common-mode interference voltage amplitude of the interference signal lines and return lines (generally referred to as signal ground), interference current flows in the loop is the loop wire and the reference object constituted.
2, the same differential mode interference current interference amplitude guide line, in the opposite direction.
