- Condition: New
- Brand: CHESHING
- Model: RS485 card
It can communicate with RDU-A through 485 protocol, send out communication signals, monitor the operation of UPS and air conditioner, monitor the operation of the alarm and the machine, control the automatic shutdown, and automatically start the call.
RS485 communication protocol RS-232 and RS-422 conversion between the principles and connections Prefix = o ns = 'urn: schemas-microsoft-com: office: office'
RS-232, RS-422 and RS-485 three kinds of interface control. The following interface standards on the serial port and the use and use of the serial port, the main use of the serial interface, External plug-ins and cables are discussed.
RS485 communication protocol The RS-232, RS-422 and RS-485 standards only specify the electrical characteristics of the interface and do not involve connectors, cables, or protocols, and users can establish their own high-level communication protocols, such as video servers With multiple RS422 serial communication interface, each interface RS422 communication lines can be controlled by an external computer to achieve recording and playback.In addition to providing a variety of video server hardware interface control, but also provides protocol interfaces, such as RS422 interface, in addition to supporting RS422 The profile protocol, but also to support Louth, Odetics, BVW and other control through the RS422 protocol.
RS-232, RS-422 and RS-485 are serial data interface standards, are developed and published by the Electronics Industry Association (EIA), RS-232 was released in 1962. RS-422 by the RS-232 development RS-422 defines a balanced communication interface that increases the transmission rate to 10 Mbps, extends the transmission distance to 4000 feet (at speeds less than 100 Kbps), and allows for a short, The RS-422 is a single-machine, multi-machine receiver, unidirectional, balanced transmission specification, named TIA / EIA-422-A standard.For extended range of applications, EIA And in 1983 based on the RS-422 developed RS-485 standard, an increase of multi-point, two-way communication capability, which allows multiple transmitters connected to the same bus, while increasing the transmitter's drive capability and sudden protection Features, extended the bus common-mode range, named after the TIA / EIA485-A standard.
RS485 communication protocol 1. RS-232 serial interface standard
RS-232 is defined as a single-ended standard for increasing the communication distance in low-speed serial communications. RS-232 takes the imbalance between the RS-232 and RS-232, which is the most widely used serial communication interface in the PC and communications industries. The typical RS-232 signal oscillates between positive and negative levels, and when sending data, the positive level of the output of the driver at the transmitter is in the range of + (+) to + 5 ~ + 15V, negative level in the 5 ~ -15V level.When no data transmission, the line is TTL, from the beginning to send data to the end of the line level from TTL level to RS-232 level and then return
TTL level. The typical operating level of the receiver is between + 3 ~ + 12V and -3 ~ -12V. Since the difference between the transmit level and the receive level is only around 2V to 3V, the common mode rejection is poor. RS-232 is designed for point-to-point (ie, only a pair of receiving and sending equipment) communications, the driver load is the maximum load capacity of 20Kbps, 3kΩ ~ 7kΩ. So RS-232 suitable for local communication between devices.
RS485 communication protocol 2. RS-422 and RS-485 serial interface standard
(1) balanced transmission
RS-422, RS-485 and RS-232 are not the same, the data signal using differential transmission, also known as balanced transmission, which uses a pair of twisted pair, which is defined as A line, the other line is defined as B. , The positive level between the transmit drivers A and B is in the logic state between + 2 and + 6V, and the negative level is in the other logic state between -2V and 6V. There is another signal ground C, Is enabled and disabled in RS-422. The 'enable' terminal is used to control the disconnection and connection of the transmit driver and the transmission line. When the 'enable' terminal is active, The transmit driver is in a high-impedance state, called the 'third state', ie it is a third state different from the logic '1' and '0'.
(2) RS-422 electrical requirements
As the receiver uses high input impedance and send the driver stronger than the RS232 drive capability, it allows the same transmission line to connect multiple receiving nodes, up to 10 nodes. That is, a master device (Master), and the rest from the device RS-422 Four-Wire Interface Because of the separate transmit and receive channels, the RS-422 does not have to control the direction of the data, and any necessary inter-device communication between the devices is necessary. The signal exchange can be implemented in software mode (XON / XOFF handshaking) or in hardware mode (a single pair of twisted pairs). The RS-422 has a maximum transmission distance of 4000 feet (1219 m) and a maximum transfer rate of 10 Mbps. The length of the balanced twisted pair is inversely proportional to the transmission rate, and the maximum transmission distance can be achieved only at 100Kbps or less. The highest transmission rate is obtained only over a short distance. Generally, a 100-meter-long twisted pair The maximum transmission rate of only 1Mbps. RS-422 requires a terminating resistor, requiring its resistance is about equal to the characteristic impedance of the transmission cable in the moment distance transmission without termination resistance, which is generally less than 300 meters without termination resistor The termination resistor is connected to the far end of the transmission cable.
(3) RS-485 electrical requirements Since RS-485 RS-422 is based on the development from the RS-485 many electrical regulations and RS-422 similar to the use of balanced transmission, need to be in the transmission line RS-485 bus, in the required communication distance of tens of meters to a kilometer, the widely used RS-485 RS-485 bus can be used two-wire and four-wire, two-wire system can realize real multi- 485 serial bus standard RS-485 uses balanced transmission and differential reception, therefore has the ability to suppress common mode interference.With the bus transceiver with high sensitivity, can detect as low as 200mV voltage, so the transmission signal can be in kilometers away RS-485 uses half-duplex mode of operation, at any time only one point in the sending state, so the sending circuit to be controlled by the enable signal RS-485 for multi-point interconnection is very convenient, you can save RS-485 can be networked to form a distributed system, which allows up to 32 drives in parallel and 32 receivers. RS-485 and RS-422 is also different in its common-mode output voltage is different, RS RS-485 is between -7V and + 12V, while RS-422 is between -7V and + 7V; RS-485 meets all RS-422 specifications, so RS-485 drivers can be used in RS-422 networks . RS-485 and RS-422, the maximum transmission distance of about 1219 meters, the maximum transmission rate of 10Mbps. Balanced twisted pair length and transmission rate is inversely proportional to the speed of 100Kbps or less, it is possible to use the provisions of the longest cable Length. Only in a very short distance to get the highest rate of transmission. General 100 meters long twisted-pair maximum transfer rate of only 1Mbps.
(4) RS-422 and RS-485 network installation points
RS-422 can support 10 nodes, RS-485 supports 32 nodes, so multi-node network.Network topology generally adopts the terminal-matching bus structure, does not support ring or star network.In the construction of network, A few points:
* Use a twisted pair cable for the bus, the nodes in series, from the bus to each node of the lead-length should be as short as possible so that the reflection of the lead signal on the bus signal the lowest impact.
* Note that the continuity of the characteristic impedance of the bus, the reflection of the signal will occur in the impedance of the discontinuous point.The following situations are easy to produce this discontinuity: different segments of the bus using different cables, or a section of the bus Too many transceivers are installed close together, and too long branch lines lead to the bus. In short, should provide a single, continuous signal path as a bus