Homemade Passive RS232-485 Converter, RS232-485 Converters
Keywords: RS232-485 converter, RS422-485 converter, MAX232, MAX485, conversion circuit, RS232 circuit, RS485 circuit
1. Brief introduction of RS-232, RS-422 and RS-485, RS-232, RS-422 and RS-485 are serial data interface criteria, which is initially developed and released by the Electronics Industry Association (EIA). RS-232 is released in 1962, named EIA-232-E, as industrial standards to ensure compatibility between products of different manufacturers. Its transmission distance is approximately 15 meters, the highest rate is 20kb / s, and the RS-232 is designed for point-to-point (ie, only one pair of receiving, sending devices). Therefore, RS232 is only suitable for local communication. RS-422 is developed by RS-232, which is proposed to make up for the deficiencies of RS-232. In order to improve the shortcomings of the RS-232 communication distance, low rates, RS-422 define a balanced communication interface, raising the transmission rate to 10MB / s, and the transmission distance is extended to 1200 meters (when the rate is less than 100kb / s), And allow up to 10 receivers on a balance bus. RS-422 is a single-way, multi-machine received one-way, equilibrium transmission specification, is named TIA / EIA-422-A standard. To extend application, EIA has developed RS-485 standards on the RS-422 in 1983, increasing multi-point, two-way communication capabilities, allowing multiple transmitters to be connected to the same bus, while increasing transmitters The driving capability and conflict protection characteristics extended the bus common mode, and then named TIA / EIA-485-A standard. Since the recommendation standard proposed by EIA is prefixed as RS, it is still accustomed to the RS pre-proportion of RS in the communication industrial field. RS-232, RS-422 and RS-485 standards are only specified for electrical characteristics of the interface without involving connector, cable or protocol, and users can establish their own high-level communication protocols on this basis. Second, homemade RS232-485 converter circuit diagram:
The RS232-485 converter mainly includes power supplies, 232 level conversion, and 485 circuits. The 232 level conversion circuit of the circuit uses NiH232 or can also be used directly using the MAX232 integrated circuit, and the 485 circuit is used by the MAX485 integrated circuit. To use convenience, the power part is designed to pass through passive mode, and the power supply of the entire circuit directly is stealing from DTR (4 feet) and RTS (7 feet) in the RS232 interface of the PC. The PC serial port can provide approximately 9mA of current per line, so the current supplied by the two lines is sufficient to supply this circuit. Experiment, this circuit only uses one of the lines to work. Use this circuitry to note that the PC program must make the serial port DTR and RTS output high. After D3 regulation, VCC is obtained, after actual test, the VCC voltage is about 4.7V. Therefore, the role to say that D3 in the circuit is that the voltage is not as good as a limiting function. The MAX485 controls the input and output of data by two pin RE (2 feet) and DE (3 feet). When the RE is low, the MAX485 data input is valid; when DE is high, the MAX485 data output is valid. In half-duplex, these two boats can usually be connected directly, and then the high and low level output by the PC or the microcontroller can make the MAX485 are converted between the received and transmit states. Since both the circuit DTR and RTs are used for circuit power, another channel and Q1 of the HIN 232 are used to control the state switching of the MAX485. The 9 pin of the usual NiH232 outputs high level. After the Q1 is inverted, the MAX485's RE and DE are low to be in the data reception state. When the PC transmits data, the 9 pin output of NiH 232 outputs a low level. After the Q1 is inverted, the RE and DE of the MAX485 are high and in the data transmission state.
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