RS232 vs RS422 vs RS485
1. RS232 (3-wire, full-duplex, single-ended, 15m cable limit)
최대 전송거리 15m, 최대속도 20 kb/s
1개의 신호전송에 케이블이 한가닥만 필요. (e.g. TX or RX) + Common gnd 필요.
1:1 전송만 가능 (Point to Point)
RS232 was developed in the 1960s, and among other things, specified an electrical standard, a protocol standard, handshaking, and connector pin-out. In general, many current applications for RS232 use only the electrical standard (3-wires, TDX, RXD, Common) and connector pin-out. While handshaking is still with us, it is usually best to disable it in software (if possible) and/or "loop-back" the pairs of signals (RTS to CTS, DTR to DSR, etc.) While RS232 was rumored to be on the "way out" with the advent of many of the new communications standards, it is still alive and well today. While the standard only supports low data rates and short line length (15m) it is still widely used and, very useful in many applications. With an external converters (RS232<=>RS485) many of the limitations of RS232 can be improved, to take advantage of, the superior properties of differential communications (2-wire or 4-wire).
2. RS422 (4-wire, full-duplex, differential, multi-drop (10 nodes), communications standard)
최대 전송거리 1.1km, 최대속도 10Mb/s
1개의 신호전송에 케이블이 두가닥 필요. (e.g. TX+/TX- or RX+/RX-)
Point to Point (1:1), Multi-Drop (1:n)모드 가능.
Multi-Drop 모드 : 하나의 Master에 최대 10개까지 Slave 연결이 가능하고, Master에서 어떤 Slave와 통신할지 결정 한 후 Query를 던짐.
While RS422 is comparable to RS485, it is limited to unidirectional data traffic, and is terminated only on the end of the line opposite the transmitter. One transmitter and 10 receivers are allowed on a network, with a distance limit of 3600ft. RS422 was on the market prior to RS485; however, due to loading limitations, one of the best uses of RS422 is probably in point-to-point communications, such as RS232 extension cords. By converting from single-ended RS232 to differential RS422 and then, converting back from RS422 to RS232 at the other end of the line, distance and noise immunity can be greatly improved.
3. RS485 (2-wire, half-duplex, differential, multi-drop (32 nodes), communications standard for distances up to 4000ft)
최대 전송거리 1.2km, 최대속도 10Mb/s
일반적인 RS485는 Half-Duplex 방식을 사용하며 2-wire로 송수신이 가능 (TRX+ / TRX-)
1:1, 1:n 이외에 Daisy-chain을 통해 여러개의 Master를 묶을 수 있음.
TX, RX를 분리한 4-wire 방식도 사용 가능.
The RS485 standard addresses the problem of data transmission, where a balanced (differential) transmission line is used in a multi-drop (party line) configuration (or point-to-point if only two devices are on the network). Up to 32-nodes (drivers and receivers) are allowed on one multi-drop, bi-directional network. Data rates of up to 10M bps are supported over short distances (12m.). At the 4000ft distance limit, data rates of up to 100K bps are allowable. RS485 specifies a 2-wire, half-duplex communications bus. Because there are differences between RS485 and RS422(minor in many instances, except for loading [12K vs. 4K) many people refere to 4-wire RS485. While RS485 is a 2-wire standard, it does offer 32 nodes on a network, on the other hand RS422 (a 4-wire standard) only specifies up to 10 nodes. Therefore, while not technically correct, it does make some sense to refer to a 4-wire RS485 network that would extend the number of nodes on a 4-wire network to 32 standard loads.
The RS485 standard only specifies electrical characteristics of the driver and the receiver, it does not specify or recommend any protocol. Because matters of protocol are left to the user, it is often difficult (if not impossible) to connect RS485 devices from different manufacturers on the same network.
The RS485 standard allows the user to configure inexpensive local networks and multidrop communications links using twisted pair wire. A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line.
Twisted pair wire with a characteristic impedance of 120 ohms is recommended with 120 ohm termination at each end of the communications line. The common-mode voltage range is -7V to +12V. A driver in the high impedence (off) state is able to remain in this state over the common mode range, whether power is applied or not. The receiver is able to respond to differential signal levels of 200mV over the common mode range. The receiver load impedence is 12K ohms (or higher) and transmitter "leakage" current is ±100µA (or less) in either the powered or unpowered state. Unloaded driver output differential voltage can be as high as ±6V. Loaded driver voltage (32 nodes on the network and termination) should typically exceed ±1.5V.
4. Converters (RS232<=>RS485, RS232<=>RS422, RS485<=>RS422)
Converters in general can be used to change the electrical characteristice of one communications standard into another, to take advantage of the best properties of the alternate standard selected. For example, an Automatic RS232<=>RS485 converter, could be connected to a computer's RS232, full-duplex port, and transform it into an RS485 half-duplex, multi-drop network at distances up to 4000ft. Converters in most instances, pass data through the interface without changing the timing and/or protocol. While the conversion is "transparent" the software must be able to communicate with the expanded network features. An "Automatic Converter" (RS232<=>RS485) will turn on the RS485 transmitter when data is detected on the RS232 port, and revert back into the receive mode after a character has been sent. This avoids timing problems (and software changes) that are difficult to deal with in typical systems. When full-duplex is converted into half-duplex only one device at a time can transmit data. Automatic Converters take care of the timing problems and allow fast communications without software intervention.
최대 전송거리 15m, 최대속도 20 kb/s
1개의 신호전송에 케이블이 한가닥만 필요. (e.g. TX or RX) + Common gnd 필요.
1:1 전송만 가능 (Point to Point)
RS232 was developed in the 1960s, and among other things, specified an electrical standard, a protocol standard, handshaking, and connector pin-out. In general, many current applications for RS232 use only the electrical standard (3-wires, TDX, RXD, Common) and connector pin-out. While handshaking is still with us, it is usually best to disable it in software (if possible) and/or "loop-back" the pairs of signals (RTS to CTS, DTR to DSR, etc.) While RS232 was rumored to be on the "way out" with the advent of many of the new communications standards, it is still alive and well today. While the standard only supports low data rates and short line length (15m) it is still widely used and, very useful in many applications. With an external converters (RS232<=>RS485) many of the limitations of RS232 can be improved, to take advantage of, the superior properties of differential communications (2-wire or 4-wire).
2. RS422 (4-wire, full-duplex, differential, multi-drop (10 nodes), communications standard)
최대 전송거리 1.1km, 최대속도 10Mb/s
1개의 신호전송에 케이블이 두가닥 필요. (e.g. TX+/TX- or RX+/RX-)
Point to Point (1:1), Multi-Drop (1:n)모드 가능.
Multi-Drop 모드 : 하나의 Master에 최대 10개까지 Slave 연결이 가능하고, Master에서 어떤 Slave와 통신할지 결정 한 후 Query를 던짐.
While RS422 is comparable to RS485, it is limited to unidirectional data traffic, and is terminated only on the end of the line opposite the transmitter. One transmitter and 10 receivers are allowed on a network, with a distance limit of 3600ft. RS422 was on the market prior to RS485; however, due to loading limitations, one of the best uses of RS422 is probably in point-to-point communications, such as RS232 extension cords. By converting from single-ended RS232 to differential RS422 and then, converting back from RS422 to RS232 at the other end of the line, distance and noise immunity can be greatly improved.
3. RS485 (2-wire, half-duplex, differential, multi-drop (32 nodes), communications standard for distances up to 4000ft)
최대 전송거리 1.2km, 최대속도 10Mb/s
일반적인 RS485는 Half-Duplex 방식을 사용하며 2-wire로 송수신이 가능 (TRX+ / TRX-)
1:1, 1:n 이외에 Daisy-chain을 통해 여러개의 Master를 묶을 수 있음.
TX, RX를 분리한 4-wire 방식도 사용 가능.
The RS485 standard addresses the problem of data transmission, where a balanced (differential) transmission line is used in a multi-drop (party line) configuration (or point-to-point if only two devices are on the network). Up to 32-nodes (drivers and receivers) are allowed on one multi-drop, bi-directional network. Data rates of up to 10M bps are supported over short distances (12m.). At the 4000ft distance limit, data rates of up to 100K bps are allowable. RS485 specifies a 2-wire, half-duplex communications bus. Because there are differences between RS485 and RS422(minor in many instances, except for loading [12K vs. 4K) many people refere to 4-wire RS485. While RS485 is a 2-wire standard, it does offer 32 nodes on a network, on the other hand RS422 (a 4-wire standard) only specifies up to 10 nodes. Therefore, while not technically correct, it does make some sense to refer to a 4-wire RS485 network that would extend the number of nodes on a 4-wire network to 32 standard loads.
The RS485 standard only specifies electrical characteristics of the driver and the receiver, it does not specify or recommend any protocol. Because matters of protocol are left to the user, it is often difficult (if not impossible) to connect RS485 devices from different manufacturers on the same network.
The RS485 standard allows the user to configure inexpensive local networks and multidrop communications links using twisted pair wire. A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line.
Twisted pair wire with a characteristic impedance of 120 ohms is recommended with 120 ohm termination at each end of the communications line. The common-mode voltage range is -7V to +12V. A driver in the high impedence (off) state is able to remain in this state over the common mode range, whether power is applied or not. The receiver is able to respond to differential signal levels of 200mV over the common mode range. The receiver load impedence is 12K ohms (or higher) and transmitter "leakage" current is ±100µA (or less) in either the powered or unpowered state. Unloaded driver output differential voltage can be as high as ±6V. Loaded driver voltage (32 nodes on the network and termination) should typically exceed ±1.5V.
4. Converters (RS232<=>RS485, RS232<=>RS422, RS485<=>RS422)
Converters in general can be used to change the electrical characteristice of one communications standard into another, to take advantage of the best properties of the alternate standard selected. For example, an Automatic RS232<=>RS485 converter, could be connected to a computer's RS232, full-duplex port, and transform it into an RS485 half-duplex, multi-drop network at distances up to 4000ft. Converters in most instances, pass data through the interface without changing the timing and/or protocol. While the conversion is "transparent" the software must be able to communicate with the expanded network features. An "Automatic Converter" (RS232<=>RS485) will turn on the RS485 transmitter when data is detected on the RS232 port, and revert back into the receive mode after a character has been sent. This avoids timing problems (and software changes) that are difficult to deal with in typical systems. When full-duplex is converted into half-duplex only one device at a time can transmit data. Automatic Converters take care of the timing problems and allow fast communications without software intervention.
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