USB and RS232 line sharing with a mux

russ · ‎2008-06-19

Posted on June 20, 2008 at 02:58

russ · ‎2008-06-19

Posted on June 19, 2008 at 14:06

I only have one usb port on the device I am designing and I really need RS232 to come out on it along with USB. I'm trying to figure out a way to allow this either by muxing or selectively enabling/disabling the MAX232 chip. The data is going out over a USB cable. Anyone have experience with this?

This is the closest mux I could find, but it doesn't have have a low enough negative voltage throughput to allow the RS232 signal:

http://www.intersil.com/data/fn/FN6515.pdf

Is it possible run USB lines through a FET (note: I only need low-speed USB)

16-32micros · ‎2008-06-19

Posted on June 19, 2008 at 18:38

Hi russ1,

Be carefull STR7 USB are only Full Speed and Not low-speed.

ST1.

kleshov · ‎2008-06-19

Posted on June 20, 2008 at 02:58

It's not easy to find an analog multiplexer that would accomodate the range of RS-232 and have sufficiently low ON-resistance. Have a look at this one:

http://www.analog.com/en/prod/0,,768_836_ADG608,00.html

It has ON-resistance of 22 Ohms, which might be good for USB, and voltage range of +/- 5 V. If you limit the voltage range of RS-232 to +/- 5 V, you could get away with this mux. And you must hope that +/- 5 V is enough for the RS-232 receiver on the other end (it should be fine, though.)