RS485 and RS232, RS422 standard and application

pressure gauge RS – 422 can support 10 nodes, RS – 485 support 32 nodes, so much of network node. Network topology usually use terminal matching of total linear structure, does not support ring or star network. In constructing the network, should pay attention to the following:
RS422 bus, RS485 and RS422 circuit principle is basic and same, it is a differential mode sending and receiving, do not need to digital ground. Differential work with rate is under the condition of the root cause of the transmission distance, this is the fundamental difference between them and RS232, because RS232 is single-ended input/output, duplex work needs at least digital ground, send line and accept line three line (asynchronous transmission), also can be added other functions such as complete synchronization control line. RS422 through the two pairs of twisted pair can full duplex work to send and receive each other is not affected, and RS485 can only half duplex work, hair accept not simultaneously, but it only need a pair of twisted-pair cable. Above three kinds of interface each have shortcomings, in the practical work can flexibly according to you need choose.
* DB9 (9 feet socket outlet), as shown in figure 2. 9 needle serial port function such as table 1.
Com is a kind of interface standard, which determines the interface electrical standards, simple physical said only a standard. No provisions interface card cable and the use of agreement, so as long as we use interface card cable accord with serial standard can in practice, flexible use, in serial interface standards to use all kinds of communication protocol and equipment control.
RS – 422, the RS – 485 and RS – 232, data signal the differential transmission mode, also called balance transmission, it USES A pair of twisted pair, will be one of the line is defined as A, another line defined as B. Normally, send drive A and B are level between in + 2 ~ + 6 v, is A logic state, negative level in – 2 v ~ 6 v, is another logic state. Otherwise a signal ground C, in the RS – 485 have a \”that can\” end, but in the RS – 422 this is available but unused. \”That can\” end is used to control sending drive and transmission line cutting and connection. When the \”that can\” end work, send drive in a high impedance state, called \”the third state\”, namely, it is different from the logic \”1\” and \”0\” the third state.
(3) RS – 485 electric regulation
A serial port control
RS – 232, the RS – 422 and RS – 485 standard docking port only the electrical characteristics make regulations, and does not involve connectors, cable or agreement, and based on this, the user can set up their own top communication protocol. For example, video server contain multiple RS422 serial communication interface, each interface all can through the RS422 communication line by external computer control to realize record and play. Video server in addition to provide all kinds of control hardware interface outside, also provide agreement interface, such as RS422 interface in addition to supporting RS422 Profile of the agreement, also support the Louth, Odetics, BVW etc through the RS422 control protocol.
Two serial socket type and conversion
Due to the receiver using high input impedance and send drive than RS232 stronger driving ability, so allow in the same on transmission line connecting multiple receiving node, most may meet 10 node. That is a main equipment (Master), the rest from equipment (Salve), from equipment can communication between, so the RS – 422 support point to many two-way communication. RS – 422 four wire interface due to using a separate the sending and receiving channel, so don’t control data direction, each device must be between any of the signal exchange all can by software methods (XON/XOFF shake hands) or hardware methods (a pair of single twisted-pair cable) to realize. RS – 422 maximum transmission distance for 4000 feet (1219 meters), the maximum transmission rate for 10 MBPS. The balance twisted-pair cable length and transmission rate is inversely proportional, in 100 KBPS rate the following, is it possible to achieve maximum transmission distance. Only in a short distance to obtain the highest rate transmission. General 100 meters long twisted pair on can get the biggest transmission rate is only 1 MBPS. RS – 422 need a terminating resistance, ask its resistance is about equal to the characteristic impedance of the transmission cable. In the moment distance transmission not be terminating resistance, namely is in commonly 300 m below does not need to terminating resistance. Terminating resistance meet in a transmission cable farthest end.
Due to the RS – 485 from RS – 422 on the basis of the development, so the RS – 485 many electric regulation and RS – 422 similar. If all the use of balanced transmission mode, all need in transmission line connected to the terminating resistance, etc. RS – 485 can use two line and the four line way, two wire can realize real multipoint two-way communication. RS – 485 bus, when asked to communication distance to dozens of meters to thousands of meters, widely used RS – 485 serial bus standard. RS – 485 the use of balanced send and receive difference, so has the inhibition common-mode interference ability. Plus bus transceiver apparatus has high sensitivity, can detect low to 200 mv voltage, the transmission signal can recover in kilometers. RS – 485 using half duplex mode, any time can only have a bit in a transmission mode, therefore, to send circuit should be made by enabling signal to control. RS – 485 for multipoint interconnection very convenient, can dispense with many signal wire. Application of RS – 485 can be networking constitute distributed system, which allows the most parallel 32 table drive and 32 sets receiver. RS – 485 and RS – 422 different also lies in its common mode output voltage is different, RS – 485 is – 7 v to + 12 v between, and RS – 422-7 v to + 7 between v; RS – 485 meet all RS – 422 standard, so the RS – 485 drive can be used in RS – 422 network application. RS – 485 and RS – 422, the maximum transmission distance is about 1219 meters, maximum transmission rate for 10 MBPS. Balance twisted-pair cable length and transmission rate is inversely proportional, in 100 KBPS rate the following, can only use regulation the longest cable length. Only in a short distance to obtain the highest rate transmission. General 100 meters long twisted pair maximum transmission rate is only 1 MBPS.
Four com in broadcast control system, the application
The expansion of the three serial port
* and a jack is RJ45, such as hubei platform use PinNiGao mss1600, mss700 video server decoding plate control every mouth for serial ports, RJ45 socket is, and broadcast control machine serial jack is DB9, so we need to use conversion cable. Convert the cable stitch definition as shown in figure 4.
The expansion of the serial port, we know the general a computer has two serial port, and for a broadcast control computer need to control equipment is far more than two devices, we need and the control of video server, video, switching platform, subtitles machine and other equipment. So we need to expand serial port, we can use the serial port expansion card in serial to expand, for instance, in the broadcast control system used in serial port expansion card MOXA CI – 134.
1. S – 232 serial interface standard
2. RS – 422 and RS – 485 serial interface standard
RS – 232, the RS – 422 and RS – 485 are serial data interface standard, are made by electronic industries association (EIA) develop and release, RS – 232 released in 1962. RS – 422 by the RS – 232, in order to improve the RS – 232 communication distance is short, low rate of faults, RS – 422 defines a balance communication interface, the transmission rate increased to 10 MBPS, transmission distance extended to 4000 feet (rate below 100 KBPS), and allow a balance bus connection a maximum of 10 receiver. RS – 422 is A kind of single sending and receiving machine of unidirectional, balance transmission standard, was named the TIA/EIA – 422 – A standard. To expand the scope of application, and EIA in 1983 in RS – 422 basis to develop the RS – 485 standard, increased the multipoint, two-way communication ability, namely allows multiple transmitter connected to the same line of bus, but also increase the transmitter driver ability and conflict protection feature, expands the bus common model circumference, named after TIA/EIA – 485 – A standard.
The above we understand the serial port of the agreement, and we daily work to contact most is the actual some equipment external interface, how do we from shape will know that it is the interface? Make the cable stitch how to define? Above we know the serial RS – 232, the RS – 422 and RS – 485 standard docking port only the electrical characteristics make regulations, and does not involve connectors, cable or agreement, and based on this, the user can set up their own top communication protocol. From our real work met use most jack has three DB9 DB25 RJ45, above three socket card can be used as a serial jack plug-ins, can also through the cable for jack conversion between. Below we will three socket stitch definitions that three socket conversion between also as long as according to jack definition of the cable can be jumper.
MOXA CI – 134 is designed for industrial environment communication application design of RS – 422/485 four serial interface card. It supports 4 independent RS – 422/485 serial ports, in a pair of multipoint communication application under, most can control 128 equipment. In order to make the RS – 485 2 line half duplex operation more simple, every piece of CI – 134 card has data flow to the automatic control (ADDC), do not need software operation. Therefore, in the Windows application under no additional coding can control the RS – 485 half duplex agreement. To achieve industrial environment on the high reliability of product requirements, the series of products to provide alternative photoelectric isolation (2 KV) and surge protection (25 KV ESD) function.
Computer control in radio and television broadcast control system use will be more and more extensive, and along with the development of computer technology, intelligent will constantly improve, the system will be a more huge, which require us to have a more thorough understanding of computer basic theory, in order to further understand the system, maintenance system, guarantee the safe and stable operation of the system.
RS485 half duplex connection for: RX + and TX + parallel for Data +; RX – and TX – parallel for Data -. RS485 FULL duplex/RS422 connection for: FULL &gnd sub; Signal wire for RX +, TX +, RX -, TX -, can according to need to GND connection.pressure gauge
(4) the RS – 422 and RS – 485 network installation note points
Now TV broadcast hard disk and industrial monitoring system used widely, the computer automatic control become mainstream. Below we through the television broadcast control introduce relevant knowledge of serial port. Other applications similarly. Video equipment control interface, and the condition monitoring interface is necessary for the induction. Computer control are mainly serial, parallel, USB, IEEE1394, Ethernet interface, etc., in the digital television broadcast center mainly adopts serial ports, Ethernet interface to control.
We know that serial RS232 effective transmission distance for 15 meters, the RS – 422 maximum transmission distance for 4000 feet (1219 meters), the maximum transmission rate for 10 MBPS. We use video broadcast control such as DVCPRO, IMX control interface have RS232, RS422 multiple interfaces for choice, if you use pin9 is RS422 interface, video server decoding mouth control are RS422 interface, but jack for RJ45 not DB9, need conversion cable for conversion. So we in the control according to the above characteristics can be flexible use, we prepared the main control switch needs, and the distance of the consider unified choose RS422 rearrangement of switch to rotate, control RS422 rearrangement of switch for the RS232 control interface, this directly by the broadcast machine itself COM port to control the rearrangement of switch to rotate, other control video, switching platform, video server decoding card use MOXA card extended RS422 interface into RS422 rearrangement of switch to rotate. Control diagram as shown in figure 5.
* should pay attention to bus characteristic impedance of continuity, the impedance discontinuity point will happen signal reflection. The following several ways easy to produce this kind of discontinuity: bus the different stages of the adopted different cable, or a period of bus have too many transceiver tight together installation, moreover is long branch line extraction to the bus.
(1) balance transmission
We usually for video server, video, switching table and direct broadcast, switch control main use serial ports, mainly used to RS – 232, the RS – 422 and RS – 485 three interface control. The serial interface standards and use and external card and cable is discussed.
The * a twisted-pair cable for bus, each node stringing up, from the bus to each node lead length should be short, so that lead the reflection in the signal to the bus signal influence the lowest.
The product features include: can choose photoelectric isolation (2 kv) and surge protection (25 kv ESD) function; Provide Data flow to the Automatic Control ADDC (Automatic Data Direction Control) function; RS – 485 data control l: ADDC or through the RTS control; Built-in terminal resistance; The chip hardware flow control, to ensure that the data is not lost; USES the advanced ASIC design, repair rate is low; Support many common operating system.
The whole system only RS422 rearrangement of open control is broadcast control machine COM (RS232) port control, and all else is MOXA card extended RS422 interface, due to the emergency switch need RS232 so in front of emergency switch with a RS422 turn RS232 converter. Through the line of broadcast control machine can to give timely broadcast control instruction, also can always read the video, switching platform, and the state of the video server. The above is only broadcast control room system control chart. Upload, total control room system control and roughly the same.
At present RS – 232 is PC machine and communication industry the most used one kind of serial interface. RS – 232 is defined as a kind of low speed serial communication adding communication distance single-ended standard. RS – 232 take unbalanced transmission mode, namely so-called single end communication. Accept, incipient data signal is relative to the signal ground. The typical RS – 232 signal in positive and negative level swing between, in sending data, sending end driver output is level in + 5 ~ + 15 v, negative level in – 5 ~ 15 v level. When many according to transmission, online for TTL from the beginning, transmit data to the end, online level from the TTL level to RS – 232 level back to the TTL level. Receiver typical work level in + 3 ~ + 12 v and – 3 ~ 12 v. Due to send level and receiving level difference is only 2 v to 3 v or so, so the common mode rejection ability difference, plus twisted-pair cable distribution of capacitance, the maximum transmission distance about 15 meters, the highest rate of 20 KBPS. RS – 232 is to point to point (i.e. use a pair of collect, hair equipment) communication and design, which drive load for 3 k Ω ~ 7 k Ω. So the RS – 232 is suitable for local of communication between the equipment.pressure gauges
* DB25 (25 feet socket outlet), as shown in figure 3. 25 needle serial port function such as in table 2.
(2) the RS – 422 electric regulation
In a word, should provide a single, continuous signal channel as a bus.