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I'm working on a small board that will convert rs232, rs485 and rs422 to TTL that I can plug into one of those BT slave modules.  My plan is to use common buses for the logic level TX and RX signals and just leave the 3 transceivers constantly enabled.  This does mean that all transceivers will transmit the same data and RX on different receivers will collide, but I consider this acceptable as the point is to have one board that will allow me to wirelessly interface with (virtually) any single serial device.

 

When I design the buses, can I just tie the requisite pins together, or do I need to isolate the buses from the transceivers with transistors? 

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So I've been doing some reading on push-pull drivers and from what I can see, I definitely cannot just tie the rx pins on to a bus, as when one of them pulls low with the other two driving high, there won't be enough impedance to keep the smoke in.  So I thought about building a separately driven bus and controlling with PNP transistors, but I don't know if that will pull the logic level low enough.

 

As for the TX, I should be okay, but I think I need to add some additional load to the bus.

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  • 1 month later...

Nope that's the right part.  For all intents and purposes RS422 is equivalent to full duplex RS485.  Half duplex RS485 is a single set of differential pairs.  RS422 just adds a second pair (4 wires total) with one pair handling RX and the other TX.  There are some minor differences between the two standards, but for a device like this, they shouldn't be an issue. 

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