Serial Data over copper pairs

My experience with serial data has shown that it doesn't go very far on its own (about 25m). My suggestion, which I have used many times in the past is to use a serial to RS485 coverter at both ends of the line. One is used to convert the RS232 signal to a RS485 signal, and the second to convert it back at the other end. These converters are relatively cheap, robust, and only use one wire pair. They can be configured to work both ways along the line. I have never had any interference using them, even when using a pair within a multicore cable setup. I have used these setups for cable runs of over 400m.

A brochure for the converter that I use can be found at http://www.alfatron.com.au/brochures/a450_brochure.PDF

I am sure that others have their own way of dealing with these issues, however I hope this helps.

Regards,
George.

The IEEE spec for RS-232 is 50 feet. Obviously it goes further under many circumstances, but you take your chances - especially using pairs on a wire carrying other stuff.

RS-232 is kind of a crappy design for any comms beyond desktop distances, because it involves detecting unbalanced 10v signals. The voltage is either there or it's not, so it's not enhanced by good wire, and is very susceptible to noise. RS-485, as my good friend George expounds in the previous post, is balanced, and therefore will go great distances with properly balanced and gauged wire, regardless of noise.

In the US, Black Box is my fave vendor of cheap, reliable powered and non-powered serial line drivers. They use 2 balanced pair (Cat 5 is balanced, "jake" is not). These line drivers are actually RS-232 -> RS-485 converters. The non-powered drivers are really slick (about the size of a matchbox), however they draw their power from the CTS pin, so they will not work with an RS-232 interface that's not fully pinned.

An example of an RS-232 interface that's not fully pinned is a GAZ display. It has only RX and GD, so if you want to use a non-powered driver with a GAZ, you have to rig up a little kit box which uses a 9-volt battery to power the line driver. My esteemed colleague Ted Savage in Montreal has a nice design for this, and I have slapped a few together myself as well.

"Non-powered" is really a misnomer, they're actually bus-powered.

Using the non-powered drivers, you should have no trouble going a mile. Using the powered ones, you can get up to 10 miles. Once upon a time, during the America's Cup, I used a pair of powered Black Box line drivers to drive a matrix scoreboard in Coronado from a PC in downtown San Diego.

You can always use the powered versions, but they are much bigger (the size of 2 tennis balls) and of course require AC 120v power.

One more thing. The slower the interface speed, the farther your throughput will go. 19.2 will surely not go more than a hundred meters, even under ideal circumstances. 2400 may go 3 times that.

James is spot on with this.

We found at Lake Louise that we could send RS-485 serial data over 3000 meters with no problem, however, you'll need to terminate the twisted pair.

Don't get scared, but you should add (about) a 600 ohm resistor at each end to balance the line. While we found that it worked 'ok' without the 600 terminator, (it would drop 1 in 10 packets), it was so much better with the 600 ohm resistor (1/8 watt is OK).

All you do is attach the resistor to both leads (you'll need one at each end--the electronic term is 'in parallel'). This balances the line at each end and prevents electronic ringing.

Then again, check RS-485 line driver you buy. It may have an internal terminator that you can switch in and out.

Mike

Just an update.

For Kitz, we found some very well priced RS485 line drivers from www.rs485.com. Talk to them on the phone if your interested.

The line drivers we used on the hill are good for -40C and only cost us $45 US each.

Mike

The man at this company did not know exactly what you are referring to. A specific product id might help.

We are having good luck with the model 209 FT line drivers by Telebyte Technology.

$150.00 for the pair.

www.telebyteusa.com

The Telebyte 245 device is similar, but additionally, it is optically isolated.

http://www.telebyteusa.com/catalog/products/245.htm

To those of us who have seen timing laptops burst into flames due to ground drain off hill pairs, and have seen 100 false impulses fire off in 5 seconds due to static discharge off hill pairs in 18% humidity, opto-isolation is priority one. Ideally, there should be no physical connection between the timing bldg and the hill pairs, only optical ones.

Bear Creek is a very short race hill in a place that rarely gets bitterly cold, so you may get away with not being opto-isolated, but I wouldn't recommend it. Consider a Blinky Box (Link to Blinky Box Manufacturer and opto-isolated line drivers.

Yes, reading data received by the serial port is equally easy. To read a byte from the serial port one just needs to read the value stored in the SBUF (99h) SFR after the 8051 has automatically set the RI flag in SCON.

Thanks
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