Communication run allowable distance

[Alwayslearningelec]

Okay, I am aware that Cat 5, 6 etc, can only be run about 330'. I have a situation where there are running communication cables(Belden9842) and Belden RS-232 over 500'. DOes the same apply for these types of cables? They are being used for Em Generator controls and metering.

Thanks.

 

[hbiss]

No, it's not the same as data network CAT 5 or 6. RS-232 allowable cable length depends on the equipment. The manufacturer's literature should provide the information you need. If not, a call to them is in order.

-Hal

 

[stevebea]

Okay, I am aware that Cat 5, 6 etc, can only be run about 330'. I have a situation where there are running communication cables(Belden9842) and Belden RS-232 over 500'. DOes the same apply for these types of cables? They are being used for Em Generator controls and metering.

Thanks.

Sounds like a stretch for RS-232. RS-485 is good for up to 4000 ft. I believe.
Generally RS-232 is run for tens of meters not the kind of distance you are refering too.

 

[gar]

110401-2112 EST

RS-232 is primarily an electrical signal interface standard. It is a moderately high source impedance, unbalanced, and unterminated system. Unbalanced means ground or common is used as a reference for the signal. Unterminated means that the load impedance at the end of the transmission line does not match the characteristic impedance of the transmission line. Unterminated transmission lines cause reflected signals (reflected energy).

Read my web site page http://www.beta-a2.com/cat-5e_photo.html for some useful information so I do not have to repeat the same thing here. This is titled CAT-5E Waveforms, does not directly relate to use at Ethernet speeds, but the discussion is related to its use for RS-232 because of its low capacitance per foot.

The original RS-232 specification had a dead band in the range of +/-3 V at the destination. This meant a voltage in this range was indeterminate. At the receiving end any voltage above +3 was one state, and anything below -3 was the other state. If the driving point voltage swung between + and - 12 V, then whatever attenuation or distortion of the waveform occurred over the transmission line the signal at the destination had to be of the correct polarity and outside of the +/- 3 V band.

If you look at my photo P-9 you see that a steady state condition exists after about 60 microseconds when the transmission line is driven by a standard RS-232 driver. If you operated at 10,000 baud that would be 100 microseconds between bits and that would probably work with CAT-5E cable. However, there could be major noise problems over this distance. If Belden 8723 with its much higher capacitance per foot was used then at 1000 ft the baud rate might be limited to about 2400 or less baud, or 9600 baud would work with 200 ft.

If you want to work at 115.2 kbaud, then the cable lengths need to be about 1/12 as long.

Do not assume these distances represent real world usable distances. There are other factors and major noise problems. Then there is also the problem of damage to components at both ends resulting from ground voltage differences.

Our I232 Isolator System can provide 4000 ft range with short time 2000 V peak isolation and at 115.2 kbaud. Maybe 8000 ft at 9600 baud. Very high noise conditions will reduce the distance.

I believe the greatest problem with direct connection of RS-232 ports relates to noise and component damage.

.

 

[GeorgeB]

+1 on gar's post. And, referring to his "Then there is also the problem of damage to components at both ends resulting from ground voltage differences.", that is a real problem when there is a lightning storm which generates momentary dramatic shifts in "earth" voltages. When I actively was involved in BETWEEN BUILDING stuff like that, we always specified optical (or other effective, sometimes transformer) isolation.

 

[stevenj76]

One trick to extend the distance is to convert with adapters to USB, then run through a USB hub, then convert back.

 

[LarryFine]

If the distance is that great, it may be time to consider an RF link.

 

[petersonra]

I am not a fan of running RS232 any distance at all. If you need to run it any length, get some RS232 to RS485 adapters, or convert it to/from ethernet.

many times there is an ethernet drop already at both ends so an ethernet convertor at both ends works out pretty cheap.

 

[esobocinski]

The original RS-232 standard (1962) was limited to 50 feet. That, however, assumed relatively simple cabling and crappy drivers. It's routinely ignored, but that's the standard. Most modern drivers called RS-232 are actually now RS-423, which is upward-compatible, but are wired to RS-232. (The equipment manufacturers usually just call them "serial"). If properly cabled, RS-423 has a 1000 ft distance limit. So what limit applies to your use? Good luck finding out. Documentation is scarce.

We run serial consoles to 300 feet at 57.6 kbps over Cat5 all the time, but we're always working in tightly controlled environments with a reliable ground reference. 500 feet is pushing it. More importantly, gar's comments about the +3 volt and -3 volt dead-band references in RS-232 are absolutely right, and is a worry. In my experience, any ground differential between the generator and the other end is a bigger worry than distance.

Is it your responsibility or the customer's that the RS-232 link works? Some sort of isolator or extender (differential driver conversion) is worth looking at. There's no reason it can't go in later though if there's a problem, if you can afford the call-back.