First, the RS-485 interface features
1, the interface signal level is lower than the RS-232C, it is not easy to damage the interface circuit chip, and the level compatible with the TTL level, can be easily connected with the TTL circuit;
2, the maximum data transmission rate of 10Mbps;
3, the use of balanced drive and differential receiver combination, anti-common mode dry capacity enhancement, that is, good anti-noise interference;
4. The maximum transmission distance of the standard value of 4000 feet, in fact up to 3000 meters, while the RS-232C interface on the bus is only allowed to connect a transceiver, that is, single-station capability. While the RS-485 interface on the bus is allowed to connect up to 128 transceivers. That is, multi-station capability, so that users can use a single RS-485 interface to easily establish a device network.
Because the RS-485 interface has good anti-noise interference, long transmission distance and multi-station capability, the above advantages make it the preferred serial interface. Because RS-485 interface composed of half-duplex network, generally only two connections, so RS-485 interface are shielded twisted pair transmission. RS-485 interface connector with DB-9 9-pin plug connector, and intelligent terminal RS-485 interface with DB-9 (hole), keyboard interface with the keyboard RS-485 using DB-9 (pin).

Second, RS-232 interface features

Is one of the communication interfaces on a personal computer. Usually RS-232 interface to 9 pins or 25 pin type appears, the general personal computer will have two sets of RS-232 interface, called COM1 and COM2.

In most cases the main use of the main channel, for the general duplex communication, only a few signal lines can be achieved, such as a send line, a receiver line and a ground.

The RS-232-C standard specifies data rates of 300,600,1200,2400,4800,9600,19200 baud per second.

RS-232-C standard, the drive allows a 2500pF capacitor load, the communication distance will be limited by this capacitor, for example, with 150pF / m communication cable, the maximum communication distance of 15m; if the capacitance per meter cable , The communication distance can be increased. Another reason for the short transmission distance is that RS-232 is a single-ended signal transmission, there are common ground noise and can not suppress common mode interference and other issues, it is generally used for communication within 20m. It as a standard, has been widely used in the computer communication interface.

However, due to RS-232 interface standards appear earlier, there are inevitably deficiencies, mainly in the following four points:

(1) interface signal level value is high, easy to damage the interface circuit chip, and because the TTL level is not compatible, it is necessary to use the level conversion circuit can be connected with the TTL circuit.

(2) the transmission rate is low, in the asyn  onous transmission, the baud rate is 20Kbps; so in the "old tree 51 CPLD development board", the integrated program baud rate can only use 19200, is the reason.

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

(4) transmission distance is limited, the maximum transmission distance standard value of 50 feet, in fact, can only be used in about 50 meters.

Third, RS-422 interface features

EIA mainly defines the serial interface between the modem and the computer in terms of telecommunications.

The physical layer specification defines 37-pin (DB-37), 25-pin (DB-25) and 9-pin (DB-9) couplings and associated cables, as well as electrical characteristics such as signal types and signals on each pin Timing.

The RS-232C uses the serial connection standard of the DB-25 or DB-9 connector with a maximum cable length of 50 feet.

RS-449 defines the serial interface of the RS-422 and DB-37 connected to the RS-423 subset.

EIA-232, the well-known RS-232, defines a serial link between a data terminal device (DTE) and a data communication device (DCE). This standard is widely adopted.

RS-422 bus and RS-485 and RS-232 circuit principle is basically the same, are sent and accepted in a differential manner, do not need digital ground. Differential work is the same rate of transmission under the same distance the root cause, which is the difference between the two and RS-232, because RS-232 is a single-ended input and output, duplex work requires at least digital ground, And receive three lines (asyn  onous transmission), you can also add other control lines to complete syn  onization and other functions. The following are the same as the "

RS422 through the two pairs of twisted pair can be full duplex work transceiver does not affect each other, and RS-485 can only half-duplex work, delivery can not be carried out at the same time, but it only needs a pair of twisted pair. RS-422 and RS-485 can transmit 1200 meters at 19kpbs. With the new transceiver line can be connected to Taiwan equipment.