Here's a web site that can give you much of the
pertinent information for RS232 vs. RS422 plus some
other somewhat common serial specs.
Disclaimer: I know nothing of this particular company
good, bad or otherwise, but found the above referenced
web page to be very informative.
http://www.rs485.com/rs485spec.html
Much of the broadcast equipment that is driven by the
software that I work with is RS422 instead of RS232.
In most cases we specify PCI plug in RS422 serial
cards for the control computers running our software.
In some cases we can use an RS232 to RS422 converter
with good reliability. I doubt if you will be
particularly successful at simply connecting RS232
with RS422 without a proper level converter. Should be
able to find one of those beasts for around $60-100.
Just do a search for "RS232 RS422 level converter"
should get many hits.
Another thing to consider/watch out for is that
pin-out standards for RS422 are lacking to say the
least, the Sony VTR control standard is quite common
in the land of broadcast equipment, - but don't count
on it being universal. Bottom line is you need the
documentation for each piece of gear you have and make
the required cable accordingly.
The fundamental difference as already described below
is that RS422 is a differential signaling scheme with
+/- TX/RX pairs going both directions (4 wires) and
RS232 is ground referenced.
Duane
N9DG
--- Jim Reid <jimr.reid@verizon.net> 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
>
> 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
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