Re: [ECOS] Transceiver required for SMC2 on MPC821?

[Stephen Worthington <stephen at digitech dot co dot nz>]

** Reply to note from Jamie Guinan <guinan@bluebutton.com> Tue, 19 Oct 1999 22:29:34 -0400 (EDT)
>   
>   
> Hi all,
>   
> I'm trying to get output from the SMC2 port on my
> MBX board (MPC821).
>   
> Is it necessary to put a transceiver between the SMC2
> pins and the serial cable if you want to talk to an
> RS-232 port at the other end?  The SMC2 Tx/Rx, and GND
> pins are available at header J1, but I had no
> luck wiring them up to a serial port.  Maybe the
> 821 can't drive the RS-232 voltages?

I can not think of a processor that can directly output RS-232.  It is
probably possible, but most processors avoid having or having to
generate a +/- 12 V supply (the -12 V is the real problem).

We use the MAX207 chip (or one of its siblings) for this sort of thing -
it lots of capability (5 drivers, 3 receivers).  It should do the trick
if you have a 5 V supply available.  It does require external capacitors
though (0.1 uF), for the charge pumps that create the voltages.  See
www.maxim.com.

I do hope that you have not blown up your SMC pins by applying RS-232
voltages directly to them already though.  If there was insufficient
current available (which is not unlikely if the drivers at the other
end are really to spec), you should be OK.

--
Stephen Worthington              Telephone: +64-4-569 6764 (home)
Digi-Tech Communications Ltd                +64-4-389 8909 (work)
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stephen@inisant.actrix.gen.nz (home)    http://www.digitech.co.nz