At 12:28 PM 6/7/2003 -1000, you wrote:
>With regard to the 232 and 422 data formats, I wondered:
>
> > >Can I just now connect them together, or are they different
> > >voltage levels; thus the need for the "interface" unit
> > >previously discussed??
>
>Eric responded, in part:
>
> > I think the interface levels are different, and the proper
> > way of dealing with RS422 is with separate grounds for each
> > datapath.
> >
> > I don't know if it is necessary, however.
> >
> > RS232 is very forgiving, so I doubt you'd do any harm in connecting
> > them, but... you may have to switch 2 and 3 to get a null-modem
> > connection. As for the (B) lines mentioned in the diagram, I don't
> > know if they're necessary for RS422 or not.
>
>Believe, with some searching about have uncovered what the
>422 system is about:
>
>"A pair of wires is used to carry each signal. The data is encoded
>and decoded as a differential voltage between the two lines.
>A typical truth table for a balanced interface is as follows:
>VA-VB < -0.2v =0
>
>VA-VB > +0.2v=1
>
>As a differential voltage, in principle the interface is unaffected
>
>by differences in ground voltage between sender and receiver."
>
>So, with the set up I now have, I have omitted the B path
>
>of the differential pair! However, it seems to me, that if there
>
>is 0 voltage on A, that the bit is still 0; and if A is high, then
>
>the bit is 1, correct?? If not, I will have to go back in and
Depends if that differential pair is tied to ground anywhere. And, IIRC,
a plus voltage wrt ground on RS232 is low and minus is high... and to
confuse things even further, the data lines are inverted from the control
lines.
I'm not quite sure where I'd go from here. Rather than actually doing
research, I'd probably look at whether the signal on an idle RS232
interface's TxD line is + or - with respect to ground. Then I'd
cobble up something using maybe a small value resistor and 1N914s to
produce that same signal with respect to ground from the RS422 signal pair.
The reason I'd do that is that most RS232 drivers now allow you to "cheat"
and reference 0 volts with respect to ground as 0. GPS manufacturers
count on that.
...but that's me. I've been known to do things the hard way and break
things... so don't blame me if you try this -- I haven't! (Never forget
that you're getting advice from a lowly tech licensee!!)
73,
Eric, KB0YDN
>add the wires for the pins 14 and 16 from the hp unit's
>
>female DB25 in order to pick up the differential pair performance.
>
>I may still not understand all of this, hi.
>
>73, Jim KH7M
--
Eric F. Richards
efricha@dimensional.com
"The weird part is that I can feel productive even when I'm doomed."
- Dilbert
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