You have to reference the signal with it's counterpart. The signal should have two pins, a plus and minus. A comparator would work to convert the signal to a single ended (referenced to ground) signal.
or use a RS-422 transeiver chip (ST490AB).
Jim C
"Eric H. Johnson"
<[EMAIL PROTECTED]
ics.com> To
Sent by: "'Enhanced Machine Controller
emc-users-bounces (EMC)'"
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Subject
03/31/2008 03:04 [Emc-users] Differental Input to
AM PWM
Please respond to
"Enhanced Machine
Controller (EMC)"
<[EMAIL PROTECTED]
sourceforge.net>
Hi all,
Note: Reposted with smaller attachment.
I am trying to read an RS422 differential input and convert it to an analog
based on the duty cycle, using a Mesa m5i20 Anything I/O board. Provided
this post is under the maximum size allowed, the schematic is attached. I
really only have access to the section labeled "CABLE TO RF AMP INTERFACE
CONNECTOR", so I can't at this point directly access the Pulse Generator.
I wrote a simple component to convert PWM duty cycle to an analog (pwmtoa)
which can be represented as follows:
Bias Gain
| |
____|______|___
| |
Enable ----| |
| |---- Out
In ----| |
|_____________|
Where Enable and In are bits, and Bias, Gain and Out are floats. Under the
current implementation, it just maintains a running average of the last 100
samples and outputs the average on time (0.0 to 1.0). When using a signal
generator this component seems to be working quite well, at least for my
purposes (error +/- 1% and latency of a couple msecs, which is more than
adequate for my purposes). I set the base period and servo period out of
phase with the signal (2kz) to make sure it does not keep sampling at the
same point in the wave form.
What I don't understand is how to covert this differential signal to single
ended. I can see the differential signal on a scope (real one, not
halscope), but when I try to reference the signal, or the "not signal" to
ground I get nothing, either on the scope or as an input to the m5i20.
Can anyone enlighten me on how to read this type of signal.
Thanks,
Eric
(See attached file: RS422.jpg)
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