I wish to passively push the cable length of an RS232 connection between two computers. What is the ideal cable to use? Is bigger better (such as #16 AWG), twisted pair, shielded? Looking to go 150 feet.

Thanks
Andy

 

Yes bigger is better. The larger the conductor the less voltage drop. With the wire you stated you should have no problems going 150ft

Regards,

James Smith
Automation Department
American Castings LLC
Pryor Oklahoma
918-476-4391 V.
918-476-4321 F.

 

Hi Andy,

Heavier gauge cable is not really necessary due to the low voltages involved. Twisted pair cable is really only intended for differential signals like RS-422/485, not for RS-232. With standard grade serial cable, you are limited to around 50 feet for RS-232. There are a few good low-capacitance cables that may allow you to drive longer distances, but the potential trade off is lower data rates and decreased reliability.

A passive solution would be to use RS-232 to RS-422/485 interface converters at both ends. RS-422/485 uses differential signals and is reliable up to a distance of around 4000 feet. You can use twisted pair cabling to drive the differential signals 150 feet without any problems. Sealevel Systems offers several interface converters that would be ideal for your application:
http://www.sealevel.com/products.asp?bus_id=4

The Sealevel interface converters do not require any drivers or additional software on the host to operate. Your software communicates via RS-232 and the signal is passively converted to RS-422 for the 150 foot run. The converter at the other end converts the signal back to RS-232 for the second computer. Since you are connecting two computers, make sure one interface converter is connected using a null modem cable.

 

EIA-232 has no distance limit - just a capacitance limit per driver. In an appendix it literally says something like "if you don't know the capacitance of your cable, don't go over 45 feet". This is where the magic "50 foot" number comes from.

It is easy to find good, low-cap cable - I like the Belden 1421A family, which has low enough capacitance to go about 55 meters. Again, here's an App note I wrote on use of RS-232 in industry while in Singapore (and doing oil-n-gass projects with a lot of outdoor lines...)
http://www.robustdc.com/library/an007.pdf

best regards
- LynnL, www.digi.com

 

Low capacitance. I believe that the RS-232 spec lists a max capacitance, not a max cable length. 50' is more of a general guideline, based on typical capacitance and resistance of cables. I would use a low capacitance cable with a low enough resistance to meet the EIA-232 specs. Use shielded cable, if noise is a concern. If not, consider using a good quality ethernet cable. I don't think twisted pair is that important for RS232, since it is a single ended signal. I think 150' feet is very realistic, with the proper cable. Check out Belden for tech info and products.

Bill

 

What baud rate do you want to run? Slow things down a little and you should be able to go well beyond 150 ft. I have successfully used RS232 at about 500 ft with no problem at 9600 baud using low capacitance cable in a low noise environment. Use shielded cable and the lowest baud rate possible and you should have no problems.

Jerry Miille

 

Thanks everyone,

Sounds like "low capacitance" is what I am shooting for.

I'll let you know the results.

Andy

 

Responding to James Smith's Sep 28, 10:17pm comment... in this instance bigger is not better:

LAL pointed out that the major criterion is low cable-capacitance. As cable (wire) size increases so does its capacitance.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Daah - I forgot the magic number ; 2500pf per driver. So just see what capacitance a cable has per foot or meter and that defines the 100% legal EIA-232 max length for that cable. If I remember correctly the 1421A family I've used often is 42pf/meter or literally 59 meters. Of course, as you run the cable farther the probability of grounding problems increases ... so be careful.

- LynnL, www.digi.com

 

Hi Andy,

Another approach is to build a serial cable with 9-pin -> RJ45 adaptors along with a network patch cable for a serial connection. I've used this successfully in noisy environments, could gain you some distance as well.

Scott

 

Yes, I've seen 232 work (or not) for amazing distances. In the UNIX biz we ran miles of cables in some buildings. One thing that I would mention is to add surge suppressors to the long runs and, of course, any runs that exit a building.

There are also short haul modems and other means to go even farther at need. See Black Box ($$) and Tut Systems for serial problem solvers.

Regards
cww