mbeals 74 Posted January 12, 2013 Share Posted January 12, 2013 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? Quote Link to post Share on other sites
mbeals 74 Posted January 12, 2013 Author Share Posted January 12, 2013 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. Quote Link to post Share on other sites
oPossum 1,083 Posted January 12, 2013 Share Posted January 12, 2013 http://www.ti.com/lit/ds/symlink/sn74lvc1g11.pdf Quote Link to post Share on other sites
mbeals 74 Posted January 13, 2013 Author Share Posted January 13, 2013 Beautiful. thanks Quote Link to post Share on other sites
tingo 22 Posted January 14, 2013 Share Posted January 14, 2013 What transceivers are you using? Quote Link to post Share on other sites
mbeals 74 Posted January 15, 2013 Author Share Posted January 15, 2013 Rs422: Max13448e Rs485: Max13487e Rs232: max232e tingo 1 Quote Link to post Share on other sites
tingo 22 Posted March 10, 2013 Share Posted March 10, 2013 Rs422: Max13448e Rs485: Max13487e Rs232: max232e For the RS-422: should that be MAX13488E? The MAX13448E looks to be a RS-485 full duplex transceiver. Quote Link to post Share on other sites
mbeals 74 Posted March 10, 2013 Author Share Posted March 10, 2013 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. tingo 1 Quote Link to post Share on other sites
tingo 22 Posted March 10, 2013 Share Posted March 10, 2013 Good to know - thanks. Quote Link to post Share on other sites
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