RS-232 RS-232-C is a serial physical interface standard established by the Electronic Industry Association (EIA) in the United States. RS is the abbreviation for “Recommended Standard”, 232 is the identifier number, and C indicates the number of revisions. Its full name is “Serial Binary Data Exchange Interface Technology Standard between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)”. The standard specifies the use of a 25-pin DB-25 connector, detailing the signal content for each pin and the voltage levels for various signals. Later, IBM simplified RS-232 to the DB-9 connector, which became the de facto standard. The RS-232 port in industrial control typically only uses three lines: RXD (2), TXD (3), and GND (5).
The RS-232-C bus standard has 25 signal lines, including one main channel and one auxiliary channel. In most cases, the main channel is primarily used, and only a few signal lines are needed for general duplex communication, such as one sending line, one receiving line, and one ground line.
The RS-232-C standard specifies data transmission rates of 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud per second.
The RS-232-C standard allows a driver to have a capacitive load of 2500pF, which limits the communication distance. For example, when using a communication cable with 150pF/m capacitance, the maximum communication distance is 15m; if the capacitance per meter is reduced, the communication distance can be increased. Another reason for the short transmission distance is that RS-232 uses single-ended signal transmission, which is susceptible to ground noise and cannot suppress common-mode interference, making it generally suitable for communication within 20m.
1. Characteristics of the RS-232-C Interface Standard:
(1) Uses negative logic, where logic “1” is -15V to -5V, and logic “0” is +5V to +15V.
(2) Operates in full-duplex mode
2. Due to the early emergence of the RS-232 interface standard, it inevitably has some shortcomings, mainly as follows:
(1) The signal voltage levels of the interface are relatively high, which can damage the interface circuit chips. Additionally, because it is not compatible with TTL levels, a level conversion circuit is needed to connect to TTL circuits.
(2) The transmission rate is relatively low, with a baud rate of 20Kbps during asynchronous transmission. Now, with the use of new UART chips like the 16C550, the baud rate can reach 115.2Kbps.
(3) The interface uses one signal line and one signal return line to form a common-ground transmission form, which is susceptible to common-mode interference, thus having weak noise resistance.
(4) Limited transmission distance, with a maximum standard transmission distance of 50 meters, but practically, it can only be used at around 15 meters.
(5) RS-232 only allows one-to-one communication, while the RS-485 interface allows up to 128 transceivers on the bus.
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RS-485 When the communication distance is required to be tens to thousands of meters, RS-485 serial bus is widely used. RS-485 uses balanced transmission and differential reception, thus having the ability to suppress common-mode interference. Additionally, the bus transceiver has high sensitivity, capable of detecting voltages as low as 200mV, allowing the transmission signal to be recovered even over kilometers.
RS-485 operates in half-duplex mode, meaning that only one point can be in the sending state at any time, hence the sending circuit must be controlled by an enable signal.
RS-485 is very convenient for multipoint interconnections, allowing many signal lines to be saved. By using RS-485, a distributed system can be established, allowing up to 32 drivers and 32 receivers to be connected in parallel.
In response to the shortcomings of RS-232-C, the new standard RS-485 has the following characteristics:
(1) The electrical characteristics of RS-485: logic “1” is represented by a voltage difference of +2V to +6V between the two wires, while logic “0” is represented by a voltage difference of -6V to -2V between the two wires. The signal voltage levels of the interface are lower than those of RS-232-C, making it less likely to damage the interface circuit chips, and this level is compatible with TTL levels, facilitating connection to TTL circuits.
(2) The maximum data transmission rate is 10Mbps
(3) The RS-485 interface uses a combination of balanced drivers and differential receivers, providing strong common-mode interference resistance, hence good noise performance.
(4) The maximum standard transmission distance of the RS-485 interface is 4000 feet, practically reaching up to 3000 meters.
(5) While the RS-232-C interface only allows one transceiver to be connected to the bus (single-station capability), the RS-485 interface allows up to 128 transceivers to be connected to the bus (multi-station capability), allowing users to easily establish a device network using a single RS-485 interface.
RS-422 The circuit principles of RS-422 and RS-485 are fundamentally the same, both using differential transmission and reception, and do not require a digital ground line. The differential operation is the fundamental reason for the longer transmission distance under the same rate conditions, which distinguishes both from RS-232, as RS-232 uses single-ended input and output, requiring at least a digital ground line, a sending line, and a receiving line (for asynchronous transmission), with additional control lines for synchronization and other functions.
RS-422 can operate in full-duplex mode through two pairs of twisted wires, allowing simultaneous sending and receiving, while RS-485 can only operate in half-duplex mode, meaning sending and receiving cannot occur at the same time, but it only requires one pair of twisted wires. RS-422 and RS-485 can transmit up to 1200 meters at 19kbps. With new transceiver designs, multiple devices can be connected on the line.
The electrical performance of RS-422 is identical to that of RS-485. The main difference is that RS-422 has four signal lines: two for sending (Y, Z) and two for receiving (A, B). Because RS-422 separates sending and receiving, it can transmit and receive simultaneously (full-duplex); whereas RS-485 has two signal lines: one for sending and one for receiving.
Differences Among the Three Differences:
① RS-232 is full-duplex, RS-485 is half-duplex, and RS-422 is full-duplex.
② RS-485 and RS-232 differ only in the physical protocol of communication (i.e., the interface standard); RS-485 uses differential transmission, while RS-232 uses single-ended transmission, but the communication program has little difference.
RS-232 is already equipped on PCs and can be used directly. If RS-485 communication is needed, a RS-232 to RS-485 converter can be connected to the RS-232 port without modifying the program.
Are there differences in the appearance of RS-232, RS-422, and RS-485 interfaces?
Generally, they are all DB9, but there are others; it still depends on the wiring inside to determine which type it is.
RS-232 is the standard interface with a D-shaped 9-pin connector, and the signal definitions of the connected devices are the same, defined as follows:
RS-232 only allows one-to-one communication (single-station capability)
RS-485 interface allows up to 128 transceivers to be connected on the bus (multi-station capability)
Since PCs typically only come with RS-232 interfaces, there are two ways to obtain RS-485 circuits on a PC:
(1) By using an RS-232 to RS-485 conversion circuit to convert the RS-232 signal from the PC’s serial port to RS-485 signal; for more complex industrial environments, it is best to choose products with surge protection and isolation.
(2) By using a PCI multi-serial port card, directly selecting an expansion card with output signals in RS-485 type.
The computer connects to multiple 485 devices (access control controllers) sequentially through the RS-232-RS-485 converter, using a polling method to communicate with the devices on the bus in turn.
The wiring markings are 485+ and 485-, corresponding to the linked devices (controllers) 485+ and 485-.
The communication distance: the theoretical distance from the farthest device (controller) to the computer is 1200 meters; it is recommended to keep it within 800 meters, with the best effect within 300 meters. If the distance is excessively long, RS-485 repeaters (extenders) can be purchased (please consult a professional converter manufacturer for purchase; the placement of the repeater in the bus should refer to the manufacturer’s instructions). Theoretically, using repeaters can extend the distance to 3000 meters.
Load quantity: The number of devices (controllers) that a single 485 bus can carry depends on the selection of the communication chips in the controllers and the 485 converter, generally ranging from 32, 64, 128, to 256 devices. This is a theoretical number; in practical applications, due to environmental conditions and communication distances, the load quantity may not reach the target. Weigeng Company designs controllers and converters for up to 256 devices, but it is recommended to keep the number of devices on each bus within 80.
The 485 communication bus must use twisted pairs or one pair of wires from a network cable; if ordinary wires (non-twisted) are used, interference will be significant, leading to poor communication or no communication at all.
Each controller device must be connected in series without star connections or branches. If there are star connections or branches, interference will be significant, leading to poor communication or no communication at all.
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