[Tig] DIY Cables

[Bob Friesenhahn]

On Thu, 22 Feb 2007, Andreas Wideroe Andersen wrote:
>
> RS422 cables: What are the pins to connect to where?
>
> RS232 cables
>
> Custom cables for sizing, GPI and a few other control cables for the Pogle.

Diagrams for these connectors are readily available on the Internet. 
For example, http://pinouts.ru/pin_SerialPorts.shtml

While RS232/422 are not addressed, I have found Leviton's "Wiring 
Strategies For Voice And Data Systems 
(http://www.leviton.com/OA_HTML/ibeCCtpSctDspRte.jsp?section=13755&minisite=10028) 
to be very useful for related purposes (e.g. RJ45 wiring).  If you use 
structured wiring techniques for all your cabling, then many benefits 
are obtained, including opportunity to re-purpose for future 
requirements.  The same installed wires could be used for RS232/422, 
telephone, video, and even gigabit ethernet.

> Any good advice on making these cables myself? (specs, examples, links etc) 
> I'm far from an expert, but I can do a little soldering.
>
> I have read that standard CAT5 UTP cable can be used for RS422 and RS232 
> cables because the specs are better than the requirements for RS422/232. Any 
> comments or recommendations for better suited cables?

Yes, the CAT5 UTP cable should be sufficient for most purposes.  I 
don't think that RS232 even specified the electrical properties of the 
cable since the specification is so old.  You can purchase ready-made 
ethernet patch cables of the desired length and should be able to find 
RS232 adaptor connectors at any electronic supply shop to make them 
work without any soldering.  RS422 is more rare (uses differential 
signaling) so I am not sure if RJ45 type adaptors are as readily 
available for it.

Traditionally "silver satin" cables have been used for RS232.  These 
use an RJ45 connector on each end but the wires are laid in parallel 
without any twisting and with no shield.  The advantage of "silver 
satin" cables is that they are very easy to build to length with a 
simple crimp tool.  The disadvantage is that the electrical properties 
are limited.

> About shielded and unshielded cables? I've read that shielded cables can 
> attract some noise/interference unless the shield coating is grounded. 
> Therefore I assume we won't need this type of cable and unshielded would work 
> just fine. Correct or am I way off? :-)

CAT5 (and CAT5e) UTP cable uses twisted pairs.  Since RS-232 is 
ground-referenced, it is not feasable to obtain benefit from the 
twisted pairs when CAT5 is used.  Performance is likely no better than 
with "silver satin".  Benefit is achieved with differential signaling 
(e.g. as used with RS-422) as long as the signal pairs are 
appropriately matched with the twisted pairs.

If shielded cable is used, then it should only be grounded on one end 
in order to avoid potential "ground loop" problems (current flowing 
through the shield due to ground potential differences).  The only 
exception to this rule is if the equipment uses the same ground (i.e. 
plugs into the same power outlet).  Unfortunately, RS232 is designed 
in a way which encourages ground loops since it is ground-referenced. 
RS422 is much better.

Bob
======================================
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/