Intelligent terminating resistor and control method

Abstract

The invention discloses an intelligent terminating resistor and a control method. The intelligent terminating resistor comprises an intelligent switching controller, a controllable pull-up resistor and a controllable pull-down resistor, wherein the controllable pull-up resistor comprises a switchable resistor and a parallel resistor, and the controllable pull-down resistor comprises a switchable resistor and a parallel resistor. When the intelligent terminating resistor specifically works, an output removal signal is initially set; when a low level jump happens to a TXD signal of a transmitting data wire, an output input signal is triggered; when a low level jump happens to an RXD signal of a receiving data wire, an output removal signal is triggered; the switchable resistors in the controllable pull-up resistor and the controllable pull-down resistor are controlled by the input signal and the removal signal output by the intelligent switching controller to synchronously conduct parallel input and parallel removal. According to the scheme, resistance values of the pull-up resistor and the pull-down resistor can be automatically switched according to the work status of the RS-485 communication circuit, the number of the maximum drive nodes of the self-reversing RS-485 communication circuit is greatly increased, and driving power consumption is lowered remarkably.

Description

technical field [0001] The invention relates to RS-485 communication technology, in particular to the technology of automatic direction switching in RS-485 communication. Background technique [0002] RS-485 is a widely used half-duplex serial communication bus. Before each communication, it is necessary to set the direction enable of the communication chip first. If the CPU is controlled by software setting, strict timing is required. Therefore, more and more practical circuits that automatically switch directions through hardware are used in RS-485 communication. [0003] The basic implementation method of RS-485 communication circuit with automatic switching direction is to control the transmission enable terminal and input enable terminal of the RS-485 transceiver through the serial output data line TXD end, and use the RS-485 transceiver’s output when sending low level. The driver actively sends out; when the high level is sent, the transceiver becomes the receiving st...

AI Technical Summary

Problems solved by technology

[0007] When the automatic switching method is used to control the communication direction without changing the resistance of the pull-up resistor and the pull-down resistor, when the two terminal resistors are connected, under the condition of 5V power supply, the two nodes communicate in parallel, and the amplitude of the differential signal on the terminal resistor when the output is high The value is only about 30mV, at this time it will not be possible to achieve normal communication between two nodes or a slightly more limited node network

[0008] If you want to make the amplitude of the analog differential signal obtained by dividing the voltage across the terminal resistor greater than 200mV, then in the case of 5V power supply, the sum of the equivalent pull-up resistors and pull-down resistors of the two nodes must be less than 1380Ω, that is, a single pull-up resistor, pull-down resistor The resistance value of the resistor is less than 1380Ω, simply calculated according to the common mode voltage of the output differential signal is 2.5V, regardless of the current consumption of the terminal resistor, the single driving current that the RS-485 transmitter needs to provide is still 1.8mA when outputting at low level, That is to say, SN75176A can only drive 33 such nodes at most in theory, and the maximum number of driving nodes is greatly lost.

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