Serial printer cabling problems

Posted on 2004-08-30
Last Modified: 2010-08-05
We are a reseller of POS (point of sale) equipment and routinely run cabling in bars and restaurants to establish connections between kitchen printers and a POS terminal.  Recently, we have run into a spate of problems with the connections between these two end points.  Through a process of elimination, we have concluded the cabling/cabling route to be the source of our problem.  Until recently, we were running flat-sat cabling until we encountered a problem at one of our installations.  One of our technicians replaced the flat sat cable with a small cat5 patch cable and the problems disappeared.  Since then, we have been using cat5 UTP cable in all of our installations.  However, we seem to be experiencing even more problems with the cat5 cabling.  It should be noted that cat5 is the cable the supplier of our software uses in their own installations.  However, I am not convinced of the efficacy of this cable in these types of environments (bars, restaurants).  I have consulted a previous post similar to this topic with the following suggestion:

"In general, however, CAT5 UTP cable is NOT THE BEST choice for RS-232 signals.  The difference is that RS-232 is SINGLE ENDED and 10BasteT network signals are DIFFERENTIAL.  The UTP cable is just that, UNSHIELDED TWISTED PAIR, which is excellent for DIFFERENTIAL signals but poor for RS-232 single-ended.
The best choice for RS-232 is SHIELDED multi-conductor 16 to 18 ga. stranded copper wire."

My question is, how accurate is the above statement?  Do you agree with this person's assessment of the cabling to use for RS-232 signals?  Furthermore, how sensitive is CAT5 UTP cabling to the external environment?  Our experience suggests it may be experiencing interference via adjacent electrical wiring.  Will shielded multi-conductor 18ga. stranded copper wire be able to withstand the types of interference likely to be encountered in a bar or restaurant?  Thank you for your time.



Question by:ejkjr
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Accepted Solution

LimeSMJ earned 350 total points
ID: 11933775
How far are these cable runs?  The maximum safe length for a standard RS-232 serial cable is 50 feet before you need to lower the baud rate to have a reliable connection.  The maximum run for a UTP CAT5 cable is around 150.  

If anything, a low capacitance shielded cable would be best for any kind of RS-232 signal.  Even better would be the use of individually shielded low cap pair wires - where each pair is shielded from the others.  I have not heard of anyone discounting the use of UTP over multiconductor cable...  especially considering that CAT5 cable exceeds all the recommendations in the RS-232 specification.

You can try using shielded CAT5 if you think the EMFs from the power lines are causing problems.  I do know that in some instances I have seen UTP cables being affected by the power from flourescent lighting in drop ceilings.

You might want to test the data runs with a loopback tester also.

Author Comment

ID: 11933869
These cable runs are typically under 50', but vary depending upon the situation.  We consistently set the port for a 9600 baud rate and probably could set it even lower.

All of our runs are tested with a loopback tester.


Author Comment

ID: 11934889
I have been doing some testing in the office with solid/stranded cat5 cabling.  I have two exact lengths (50') of each type of cable.  The stranded cable prints correctly at up to 19200 baud rate.  The solid cable does not even print at 9600 baud.  What characteristic of the stranded cabling causes it to print correctly?

Expert Comment

ID: 11935421
What connectors are you using for the cables?  Solid cables do not fair well with crimped connections because the solid wire break easily...
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Author Comment

ID: 11940804
Yes, we are using crimped connections on all of our cables.  I do not actually make the cabling...I am just a programmer.  So, I am not familiar with the different types of connectors.  Is there a preferable connector for solid cat5 cabling?  Thanks.

Assisted Solution

LimeSMJ earned 350 total points
ID: 11941623
Solid CAT5 should really only be used for punchdown blocks such as for telephony.
LVL 11

Assisted Solution

PennGwyn earned 150 total points
ID: 11943655
Maximum run for Cat5 carrying Etherent is 100 meters, about 330 feet.  50 feet for RS-232C sounds about right.

The "twisted pair" part of UTP is a measure taken to reduce noise and interference.  I can easily believe that it's also reducing the RS-232C signals....

The "unshielded" part of UTP is that because it uses twists to reduce noise and interference, it doesn't need shielding to keep out interference.  There may well be equipment in your environments that puts out interference, and so any non-twisted solution may need to include shielding.

If it's shielded and not twisted, it's not UTP, so you can't call it "Cat5" any more....

Stranded cable will flex better without risk of breakage.  Solid cable may be getting broken during installation if not handled correctly.  (We require our cabling contractors to use only installers with current certification for the media being installed.)

How many conductors are you actually using?  3? 7?


Author Comment

ID: 11944850
We are using 5 conductors.

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