🚀 go-pugleaf

In article <JON_SREE.92Aug18090955@world.std.com> jon_sree@world.std.com (Jon Sreekanth) writes:
>Does anyone know if RS232 drivers are normally slew rate limited ? 
>For example, looking at the 1488 specs, an inexpensive RS232 driver, 
>the inherent slew rate is more than the EIA 30 V/uS, and a 330pf 
>capacitor is recommended to be added. 
>
>Is it conventional to skip this cap, and figure there will be enough 
>cable capacitance ?

All the designs *I've* seen leave the cap out  Although if you're
working on a new design and it's "high-speed," I'd recommend to the cap
in your design and leave it unstuffed until you find you have
problems....

>What is the problem with too high a slew rate ? 
>FCC 15 problems ?

Probably mostly near-end crosstalk.  I worked on a system with an EIA
232 (actually RS 232 back then!) synchronous interface running at 9600
bps.  When the interface was left unconnected, the modem control lines
picked up cosstalk from the clock lines.  The modem control lines (on a
Zilog SIO) interrupted on every *transition*, causing quite a load on
the poor little 2 MHz Z80 driving the port.  Slew-rate limiting fixed
it.  I would imagine that FCC Part 15 could also be issue today.

>/ Jon Sreekanth

-hans
-- 
  Hans-Gabriel Ridder				Digital DECwest Engineering
  ridder@rust.zso.dec.com			Bellevue, Washington, USA
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