Mike S wrote:
On 1/8/2012 5:19 PM, unruh wrote:
...
Make sure you properly terminate the cable with 100 ohms. Otherwise you
will get reflections back and forth along the cable, and the cable will
slowly fill with charge, broadening the pulse by a huge amount. It may
also be that the signal is decreasing due to resistance in the line.
The proper termination for EIA-232 is 3000-7000 ohms.
EIA-232 deliberately mis-terminates in order to make the line
capacitive, and slow down the rise times. It also has a 50 foot limit,
to limit the capacitance and rise time. If I remember correctly -422
uses correctly terminated lines.
-232 is actually mis-terminated at both ends, but with different
impedances. If the receiving end it mis-terminated, you also have to
consider the transmitting end, which is terminated with a rather lower
impedance, but still above the line impedance.
If you want fast edges to be reproduced accurately, you must do, as
Unruh says, and terminate the line with its characteristic impedance.
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