I have a serial device I want to place about 30-35 feet from the computer. I’m told by the vendor that I can’t connect the device more than 27 feet using an RS232 cable. They said that I can run it much longer using RS422, and that the parts are easily available. So what’s the difference between RS232 and RS422? What adapters do I need to connect an RS232 device to RS422 and what sort of cabling should I use?
Here is a good page explaining the differences. In part, it says:
All the parts you’d need are pretty readily available from various vendors, such as Allied Electronics.
RS422 is needed for substantially larger distances–thousands of feet–and higher baud rates. I have personally run serial RS232 connections at ~50 feet and seen no degradation. I am assuming this is the 12v output standard, rather than the 5v, but again ~50 feet should be easily covered by RS232.
That said, an RS422 converter is fairly cheap; I found a $33 one at Pricegrabber. Cable is the same as for RS232; you can find models that take a twisted pair connection, DB9, etc. But again, I’d just try it with a long RS232 cable; if it doesn’t work, you can still use the cable and buy converters.
Yeah, go with that one. I didn’t even notice the price of the one I found!
I’ve also run RS232 cables for 50+ feet and it works fine.
What baud rate are you running?
27 feet? Heck, I’ve successfully interfaced with RS-232 devices that were 60 feet from my computer. Chances are you will not have any problems.
Also keep in mind that distance is not the most important limitation of RS-232. Grounding is much more important. Because RS-232 is a single-ended system (meaning the receiver looks at the signal relative to earth ground), it is important for the driver and receiver to share a common earth ground, and ideally a single-point ground. This is not as important with differential serial interfaces such as RS-422 and RS-485.
I don’t know what the baud rate is. The device is a handheld barcode scanner (of the sort used in a checkout counter). The manufacturer said 27 feet is the maximum length using RS232, so I’m going to pursue RS422 to get the slightly longer distance we need. I’d experiment with RS232 if I were doing this on my own, but since there’s a department that depends on this working, I think it’s better not do so.
I am really surprised that the manufacturer wants to limit yours to 27 feet, unless the baud rate is really high. I have used RS-232 at lengths of up to 50 feet in manufacturing plants, which is an electrically noisy environment, and I’ve seen people get it working at well over 100 feet. The general rule of thumb for RS-232 is 50 feet max. If you are only going 30 to 35 feet it shouldn’t be a problem at all.
If you insist on going with RS-422, I’ve used RS-232 to 422 converters from Black Box (www.blackbox.com). They are pretty reliable and work even in industrial use. I’m not sure of the cost but I suspect they cost a lot more than the one that CJJ* linked to.
With that short of a distance over RS-422 the cable isn’t going to make that much of a difference, but if you are already shelling out the bucks for the 422 converter you might as well get decent quality cable. You’ll need a shielded 4 conductor cable. The converter box should come with a wiring diagram.