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How is this RS485 module working?

rishi2628

I am using an RS485 module :- http://www.elecrow.com/uart-ttl-to-rs485-twoway-converter-p-1545.html

This module is bidirectional and uses a single pair of differential signal to transmit/receive data. My question is how is the chip deciding whether it has to transmit data from TX to A+ B- or from A+ B- to RX?. Shouldn't there be 2 pairs of differential signal, one pair for TX and the other pair for RX? What is happening in this module??

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    RS-485 is usually a half-duplex protocol with a single master and multiple slaves sharing a single A+B- differential pair.  The master sends out a "poll" request and then disables its transmitter while it waits for a reply.  The poll request includes an address, and the slave that matches the request replies to the poll, enabling its transmitter for the length of the message.  Only one device at a time drives the A+B- pair.

     

    I can't see how this module works.  Usually there's an extra TTL pin to enable the transmitter.  It's usually tied to the UART's RTS signal.  The product page does not show IC chip numbers, so you can't figure out how it works or whether the ICs are anything other than counterfeit junk.

  • 0 in reply to johnbeetem

    The clue is probably the diode in the middle. My guess would be a simple re-triggerable monostable (Schottky diode, cap and resistor) running from the transmit data to control the transceiver's direction. The large chip might just be CMOS logic buffers or inverters.

  • 0 in reply to jc2048

    Thats also entirely likey. image

    Difficult to tell with anonymous chips. Though the Data Direction connections do appear to be routed to Pin 12 on U2.

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