[TenTec] hp Z3801A GPS Unit

[Jim Reid]

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

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