Difference between Several Kinds of Communication Serial Interfaces

I. Introduction to serial interface

RS-232, RS-422 and RS-485 are serial data interface standards, which were originally formulated and issued by Electronic Industries Association (EIA). Issued in 1962, RS-232, named as EIA-232-E, is an industrial standard for ensuring compatibility between products from different manufacturers. However, RS-232 shows some weakness, such as short communication distance and low efficiency so RS-422 is developed to make up such weakness. RS-422 defines one kind of balanced communication interface that can transmit data at rates as high as 10 Mbit/s, extends the transmission distance to 4000 feet (when the rate is lower than 100kb/s) and allows connection to maximum 10 receivers on one balanced bus. RS-422, also known as TIA/EIA-422-A, is an uni-directional, balanced transmission specification that specifys one sender with multiple receivers .In order to expand the application scope, EIA formulated RS-485 standard in 1983 on the basis of RS-422. RS-485 supports multipoint and bidirectional communication, that is to say it allows multiple senders to be connected to same bus. Meanwhile, it is provided with function of sender and conflict protection characteristics, expands the scope of common-mode of bus. Later, it is renamed as TIA/EIA-485-A standard. As all recommended standards put forward by EIA are labeled with the Prefix “RS”, the communication industry is used to label above standards with this prefix “RS”.

RS-232, RS-422 and RS-485 standard only specify the electrical characteristics of the interface, and don’t involve connector assembly, cables or protocols. On this basis, users can establish their own high level communication protocol. Therefore, in the video industry, many manufacturers have established a set of high level communication protocol, public or used by manufacturers independently. For example, Sony from VCR manufacturer is different from Sony’s RS-422 control protocol against VCR and there are more control protocols on video server. For example, Louth, Odetis protocol are public while ProLINK is based on Profile. 

II. RS-232 serial interface standard 

Currently, RS-232 is one kind of serial interface, which is most widely used in PC machine and communication industry. It is defined as an single-ended standard that can increase the communication distance under low-rate serial communication. It adopts unbalanced transmission mode, i.e., single-ended communication.

The data signal at receiving terminal and transmitting terminal is relative to signal ground; for example, for the data sent by DTE, when DB25 connector is applied, the data is the level of pin-2 to pin-7 (signal ground). Typical RS-232 signal swings between positive and negative levels; when data is being sent, the drive at the transmitting terminal outputs +5 to +15V positive level and -5 to -15V negative level. When there is no data transmission, line level is TTL; from the start of data transmission to its end, line level changes from TTL level to RS-232 level, then backs to TTL level. Typical working level of receiver is  +3 to +12V and -3 to -12V. Since the difference between transmission level and reception level is only about 2V to 3V, so its common-mode suppression capacity is poor; besides, there is distributed capacitance over twisted-pair cable, so the maximum transmission distance is about 15m, with maximum rate of 20kb/s. RS-232 is specially designed for point-to-point communication(i.e., only one pair of receiving and transmitting devices); its drive load is 3 to 7kΩ. Therefore, RS-232 is suitable for communication between local devices and its electrical parameters are as shown in the table below. 

Specification RS232  RS422  R485

Working mode: Single-ended and differential  

Node number: 1 for receiving, 1for transmitting, 1 for transmitting, 10 for receiving 1 for transmitting 32 for receiving 

Maximum transmission cable length 50 feet, 400 feet, 400 feet 

Maximum transmission rate 20Kb/S 10Mb/s 10Mb/s

Maximum driving output voltage  +/-25V -0.25V-+6V -7V-+12V

Output signal level (minimum value under load) under load of driver +/-5V-+/-15V +/-2.0V +/-1.5V

Output signal level (minimum value under no load) under no load of driver +/-5V-+/-15V +/-2.0V +/-1.5V

Load impedance of driver (Ω) 3K-7K 100 54

Slew rate (maximum) 30V/μs N/A N/A

Input voltage range of receiver +/-15V -10V-+10V -7V-+12V

Input threshold of receiver +/-3V +/-200mV +/-200mV

Input resistance of receiver (Ω) 3K-7K 4K (minimum) ≥12K

Common-mode voltage of driver -3V-+3V -1V-+3V

Common-mode voltage of receiver -7V-+7V -7V-+12V

III. RS-422 and RS-485 Serial Interface Standards

1. Balanced transmission 

RS-422 and RS-485 are different from RS-232. Their data signal adopts the mode of differential transmission, also called balanced transmission; it uses a pair of twisted-pair cables, of which one cable is defined as A, while the other is defined as B. 

On general conditions, the positive level between transmitting drives A and B is +2V to +6V; this is a logic state; the negative level is -2 to 6V, indicating another logic state. There is also a signal ground C; for RS-485, there is also an Enable terminal, but this terminal is optional for use. Enable terminal is intended to control the cut and connection of transmitting drive and transmission cable. When it works, transmitting drive is under high-impedance state, called “third state”, which differs from logic “1” and “0”. 

Receiver is also defined relative to signal transmitting terminal; transmitting and receiving terminals connect AA and BB through balanced twisted-pair cable. When there is a level between receiving terminals A and B that goes above +200mV, positive logic level will be outputted; when the said level goes below -200mV, negative logic level will be outputted. The level acceptable to receiver falls into the range from 200mV to 6V usually. 

RS-422 standard, full name “Electrical Characteristics of Balanced Voltage Digital Interface Circuits”, defines the characteristics of interface circuits. Actually, it has one additional signal ground cable, totally 5 cables. Since receiver adopts high input impedance and transmitting drive that has stronger drive capacity in comparison with that of RS232, it’s allowed to connect the same transmission cable to multiple receiving nodes, up to 10 nodes totally, i.e., one Master, multiple Slaves; but slaves could not communicate with each other. Therefore, RS-422 supports the two-way point-to-many communication. Receiver’s input impedance is 4k, so transmitting terminal’s maximum load capacity is 10×4k+100Ω (terminal resistor). RS-422 four-wire interface adopts independent transmitting and receiving channels, for which, no need to control data direction, and any required signal among different devices can be realized by either software mode (XON/XOFF handshake) or hardware mode (a pair of independent twisted-pair cables). 

　For RS-422, maximum transmission distance is 4,000 feet (about 1,219m) and maximum transmission rate is 10Mb/s. Its balanced twisted-pair cable’s length is inversely proportional to transmission rate, but only under the transmission rate of 100kb/s, the cable can reach maximum transmission distance. Maximum transmission rate can be obtained only under very short distance. Generally, the maximum transmission rate of a 100m long twisted-pair cable is only 1Mb/s. 

RS-422 needs a terminal resistor; the terminal resistor’s resistance is approximately equal to the characteristic impedance of transmission cable. Under short transmission distance, terminal resistor may not be required; that is to say, terminal resistor may not be required under the length below 300m. Terminal resistor connects to the farthest end of transmission cable.

See Table 1 for related electrical parameters of RS-422

3.Electrical specification for RS-485

Since RS-485 has been developed on the basis of RS-422, so many electrical specifications for RS-485 are quite similar with that for RS-482. For example, they both adopt balanced transmission mode and need to put a terminal resistor in connection. RS-485 may adopt two-wire and four-wire systems; while using two-wire system, multipoint two-way communication can be truly realized. 

When four-way system is used, RS-485 could only only realize the point-to-many communication as RS-422; that is to say, there can be only one Master, with the others as Slaves, but it has improved from RS-422 in some way - RS-485 can be used to connect to up to 32 devices on the bus, for either four-way system or two-way system. 

Also, RS-485 differs from RS-422 in common-mode output voltage: RS-485, -7V to +12V, but for RS-422, -7V to +7V. RS-485 receiver’s minimum output impedance is 12k, while that of RS-422 is 4k; RS-485 could meet all the specifications of RS-422, so the drive of RS-485 can be applied to the network with RS-422. 

See Table 1 for related electrical parameters of RS-485. 

RS-485 is the same as RS-422: its maximum transmission distance is about 1,219m and its maximum transmission rate is 10Mb/s. Its balanced twisted-pair cable’s length is inversely proportional to transmission rate, but only under the transmission rate of 100kb/s, maximum length of cable as specified can be used. Maximum transmission rate can be obtained only under very short distance. Generally, the maximum transmission rate of a 100m long twisted-pair cable is only 1Mb/s.

RS-485 needs two terminal resistors; terminal resistor’s resistance is required equal to the characteristic impedance of transmission cable. Under transmission distance, terminal resistor may not be required; that is to say, terminal resistor may not be required under the length below 300m. Terminal resistor connects to two ends of transmission bus.