High impedance state control circuit for RS-485 transceiver

Abstract

The invention relates to a high-impedance state control circuit for an RS-485 transceiver. The high-impedance state control circuit comprises an RS-485 output driving module and a high-impedance state control module; a bus output stage of the RS-485 output driving module is electrically connected with an NPN triode and a PNP triode, and the high resistance state control module is used for enabling the NPN triode and the PNP triode to be cut off when the RS-485 output driving module is enabled to be closed. According to the high-resistance state control circuit for the RS-485 transceiver, the triode is adopted as a bus output stage and a high-resistance state control module, when the RS-485 transceiver is controlled to be in an enabling closing state within the A / B common-mode level range of -7v to 12v, the output of the RS-485 transceiver is in a high-resistance state, the ESD voltage of the A / B output port of the bus is greatly improved, and the application in most occasions is met.

Description

technical field [0001] The invention relates to an RS-485 transceiver, in particular to a high-impedance state control circuit for the RS-485 transceiver. Background technique [0002] RS-485 is a standard defining the electrical characteristics of drivers and receivers in a balanced digital multipoint system, defined by the Telecommunications Industry Association and the Electronic Industries Alliance. Digital communication networks using this standard can effectively transmit signals over long distances and in environments with high electronic noise; RS-485 makes it possible to connect local networks and configure multi-drop communication links. [0003] The RS-485 chip must be in a high-impedance state after the output drive is turned off. At this time, the voltage range it can withstand is stipulated in the protocol as -7v~12v. The existing RS-485 chip usually uses CMOS output, and the negative voltage will cause The parasitic diode of the NMOS transistor is turned on a...

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Problems solved by technology

[0003] The RS-485 chip must be in a high-impedance state after the output drive is turned off. At this time, the voltage range it can withstand is stipulated in the protocol as -7v~12v. The existing RS-485 chip usually uses CMOS output, and the negative voltage will cause The parasitic diode of the NMOS tube is turned on and thus clamped, and a high voltage input greater than the power supply voltage will cause the parasitic diode between the drain and the N well to conduct

[0004] The current solution on the market is to connect a reverse diode to the output port to block the output port, form a connection with the parasitic diode, and block the voltage clamping; this method makes the ESD voltage of the two output ports of the bus A / B very low. It is difficult to reach 15,000 volts, which limits many usage scenarios

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