RS232 rs422 RS485 details

1. RS-232-C

RS-232-C is the Electronic Industry Association (EIA)
A standard for serial physical interfaces. RS is the abbreviation of the English "Recommendation Standard". 232 indicates the ID number, and C indicates the number of modifications. The RS-232-C bus standard has 25 signal lines, including a main channel and a secondary channel.

In most cases, the main channel is used. For general duplex communication, only a few signal lines are required, such as a transmission line, a receiving line, and a ground line.

The RS-232-C standard sets the data transmission rate per second 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600 port.

RS-232-C standards stipulate that the drive can have pf capacitive load, the communication distance will be limited by this capacitance, for example, when using a 150pf/M communication cable, the maximum communication distance is 15 m; if the electrical capacity of each meter cable is reduced, the communication distance can be increased.

Another reason for short transmission distance is that the RS-232 is a single-ended signal transmission, there are common noise and can not suppress common mode interference and other problems, so it is generally used for communications within 20 m.

2. RS-485

RS-485 bus, when the communication distance is dozens of meters to kilometers, widely used RS-485
Serial Bus. The RS-485 adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. In addition, the bus transceiver is highly sensitive and can detect voltage as low as 200mV. Therefore, the transmission signal can be recovered from kilometers away. RS-485 uses half duplex mode, at any time only a little in the sending State, therefore, the sending circuit must be controlled by the enabling signal.

RS-485 for Multi-Point interconnection is very convenient, can save a lot of signal lines. Application RS-485
A Distributed System can be connected to IOT platform. A maximum of 32 drivers and 32 receivers can be connected in parallel.

3. RS-422

Rs422 bus, RS485 and rs422 circuit are basically the same principle, are sent and accepted in a differential mode, do not need a digital ground.

Differential operation is the root cause of long transmission distance at the same rate, which is the fundamental difference between the two and RS232. Because RS232 is a single-ended input and output, at least the digital ground line is required for duplex operation.
. Transmission Line and receiving line (asynchronous transmission). You can also add other control lines to complete synchronization and other functions. Rs422 can work in full duplex mode without affecting each other through two pairs of twisted pair wires, while RS485 can only work in half duplex mode, but it only needs one pair of twisted pair wires. Rs422 and RS485 can transmit 1200 meters under 19kpbs. Connect devices to the new transceiver line.

RS-232, RS-422 and RS-485 are serial data interface standards, initially by the Electronic Industry Association (EIA) developed and released, RS-232 released in 1962, named as EIA-232-E, as industrial standards, to ensure compatibility between products of different manufacturers.

RS-422 from the Development of RS-232, it is to make up for the lack of RS-232 and proposed. In order to improve the shortcomings of short RS-232 communication distance and low speed
A balanced communication interface is defined to increase the transmission rate to 10 Mb/s, extend the transmission distance to 4000 feet (when the speed is lower than kb/s), and allow up to 10 connections on a balanced Bus
Receiver.

RS-422 is a kind of single-machine transmission, multi-machine receive one-way, balanced transmission specification, named as TIA/EIA-422-A standard. In order to expand the scope of application, EIA was launched in RS-
422 based on the establishment of RS-485 standards, added multi-point, two-way communication capabilities, that is, allow multiple transmitters to connect to the same bus, while increasing the transmitter driving capacity and conflict protection characteristics, extended
Bus common mode range, named as TIA/EIA-485-A standard. Since the proposed standards proposed by EIA are prefixed with "Rs", the above standards are still used
RS is the prefix title.

RS-232, RS-422 and RS-485 standards only regulate the electrical characteristics of the interface, and do not involve connectors, cables or protocols, on the basis of which users can establish their own high-level communication protocol. Because
This is an application in the video industry. Many manufacturers have established a set of high-level communication protocols, either open or exclusively used by manufacturers. For example, the recorder manufacturers Sony and Panasonic on the video recorder RS-422 control protocol is different
More control protocols are available on the video server. For example, Louth and odetis protocols are open, while prolink is based on profile.

Ii. RS-232 Serial Interface Standard

At present, RS-232 is one of the most widely used serial interfaces in PC and communication industry. A RS-232 is defined as a single-ended standard that increases communication distances in low-rate serial communication. The RS-232 adopts the unbalanced transmission mode, that is, the so-called single-ended communication.

The data signal sent and received is relative to the signal location. For example, the data sent from the DTE device is 2-pin relative to 7-pin (Signal Location) When db25 connector is used, for the definition of db25 pins, see Figure 1. Typical
The RS-232 signal swing between positive and negative levels, when sending data, the sender drive output is equal to + 5 ~ + 15 V. The negative value ranges from-5 ~ -15 V level. When no data is transmitted
TTL, from the beginning to the end, the online level from the TTL level to the RS-232 level and then return the TTL level. The typical operating level of the receiver is + 3 ~ + 12 V and-3 ~ -12 V. Because
The difference between the transmission level and the receiving level is only 2 V to 3 V, so the common mode suppression capability is poor. In addition, the distribution capacitance on the twisted pair wires can reach a maximum transfer distance of about 15 meters, the maximum speed is 20 kb/s. RS
-232 is designed for point-to-point (that is, only one pair of receiving and sending devices) Communication, and its drive load is 3 ~ 7kb. So RS-232 is suitable for communication between local devices. For more information about the electrical parameters, see table 1.

Rules		RS232	Rs422	R485
Work Mode		Single-ended	Difference	Difference
Number of nodes		1 Receiving and 1 sending	1 send 10 receive	1 send 32 receive
Maximum length of transmission cable		50 feet	400 feet	400 feet
Maximum Transmission Rate		20kb/s	10 Mb/s	10 Mb/s
Maximum drive output voltage		+/-25 V	-0.25v ~ + 6 V	-7v ~ + 12 V
Driver output signal level (Minimum load)	Load	+/-5 V ~ +/-15 V	+/-2.0 V	+/-1.5 V
Driver output signal level (No-load maximum)	No Load	+/-25 V	+/-6 V	+/-6 V
Driver load impedance (Ω)		3 K ~ 7 K	100	54
Pendulum rate (maximum)		30 V/μs	N/	N/
Receiver Input voltage range		+/-15 V	-10v ~ + 10 V	-7v ~ + 12 V
Receiver Input threshold		+/-3 V	+/-200mV	+/-200mV
Receiver Input resistor (Ω)		3 K ~ 7 K	4 K (minimum)	≥ 12 K
Drive Common Mode Voltage		-	-3v ~ + 3 V	-1v ~ + 3 V
Common Mode Voltage of the receiver		-	-7v ~ + 7 V	-7v ~ + 12 V

Table 1

Iii. Serial Interface Standard for RS-422 and RS-485

1. Balanced Transmission

RS-422, RS-485 and RS-232 is not the same, the data signal using differential transmission mode, also known as balanced transmission, it uses a pair of twisted pair, the first line is defined as a, the other line is defined as B, 2.

Normally, the power plane between drive a and drive B is equal to + 2 ~ + 6 V is a logical state with a negative level ranging from-2 ~ 6 V is another logical state. Another Signal Location C, in the RS-485
There is also a "enable" end, which is usable in the RS-422. The enable end is used to control the disconnection and connection between the transmission drive and the transmission line. When the enable device acts, the sending drive is in high
The blocking state is called the "third State", that is, it is a third State different from the logic "1" and "0.
The receiver is also set to correspond to the sending end. The receiving and sending end are connected to the BB through the balanced twisted pair. When there is more than 200mV power between the receiving end AB, positive logic level. If the value is less than-200mV, the negative logic level is output. The level range of the receiver receiving balancing line is usually between 200mV and 6 v. See figure 3.

2. RS-422 electrical Regulations

The RS-422 standard stands for the electrical characteristics of the balanced voltage digital interface circuit, which defines the characteristics of the interface circuit. Figure 5 is a typical RS-422 4-line interface. There is actually a signal ground, a total of 5
Root line. Figure 4 shows its DB9 connector pin definition. Since the receiver adopts a high input impedance and a transmission drive with a stronger drive capability than RS232, it is allowed to connect multiple receiving nodes on the same transmission line. A maximum
Connect 10 nodes. That is, a master device, the rest is the slave device (Salve), the slave device cannot communicate, so the RS-422 supports two-way communication to multiple. Receiver Input Impedance
It is 4 K, so the maximum load capacity of the initiator is 10 × 4 K + 100 Ω (final resistance ). RS-422 4-line interface because of the use of separate transmission and receiving channel, so do not control the data direction between the device
Any necessary signal exchange can be implemented by means of software (Xon/xoff handshake) or hardware (a pair of individual twisted pair wires.

The maximum transmission distance of the RS-422 is 4000 feet (about 1219 meters), and the maximum transmission rate is 10 Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate.
To reach the maximum transmission distance. The maximum transmission rate can be obtained only within a short distance. Generally, the maximum transmission rate of a 100-meter twisted pair is only 1 Mb/s.

RS-422 needs a final resistance, the resistance value is equal to the characteristic impedance of the transmission cable. When the moment distance is transmitted, the final resistance is not required, that is, the final resistance is generally less than 300 meters. The final resistance is connected to the far end of the transmission cable.

The related electrical parameters of RS-422 are shown in Table 1

3. RS-485 electrical Regulations

Because the RS-485 is developed from the basis of the RS-422, so many RS-485 electrical regulations and RS-422 is similar. For example, the balanced transmission mode is adopted, and the final resistance must be connected to the transmission line. The RS-485 can adopt two-line and four-line mode, two-line can realize real multi-point bidirectional communication, see Figure 6.

The use of four-line connection, and the RS-422 can only achieve Point-to-many communication, that is, only one master (master) device, the rest is from the device, but it is better than the RS-422,
32 more devices can be connected to the 4-wire or 2-wire connection bus.

The difference between RS-485 and RS-422 is also that the common mode output voltage is different, the RS-485 is-7 V to + 12 V, while the RS-422 is between-7 V to + 7 V, RS-
The minimum input impedance of the 485 receiver is 12 K, and the RS-422 is 4 K; The RS-485 meets all RS-422 specifications, so the RS-485 drive can be used in RS-422 networks.

See table 1 for RS-485-related electrical requirements.

Like the RS-485, the maximum transmission distance is about 1219 meters and the maximum transmission rate is 10 Mb/s. The length of the twisted pair is inversely proportional to the transmission rate at a rate of kb/s.
The longest cable length can be used. The maximum transmission rate can be obtained only within a short distance. Generally, the maximum transmission rate of 100-meter long twisted pair wires is only 1 Mb/s.

The RS-485 requires two final resistance, and the resistance value must be equal to the characteristic impedance of the transmission cable. When the moment distance is transmitted, the final resistance is not required, that is, the final resistance is generally less than 300 meters. The final connection resistor is connected to both ends of the transmission bus.