I am using a cable of approx 1600ft to enable communication between two computers using RS-232(pins 2,3 and 5). On checking the transmitted signal on the oscilloscope, i get the output as below (hex code:41H):
However, on oscilloscope at the receiver end, following is displayed:
The connection does not involve any terminal resistances and when I checked the output by connecting the oscilloscope probe directly to the rs-232 pins, the output was as seen in the first image, however, on connecting it to the computer and then checking it on an oscilloscope, a noisy output as seen in second image was displayed.
Can someone please suggest on what should be the correct approach to enable communication using RS-232 for such long distance.
Also, the receiver computer does not show any output on transmitting any of the hex codes. The baud rate used was 300.
Thank you.
Best Answer
No, because RS-232 was never designed to work over such distances. A long cable like that picks up a lot of noise and interference, like an antenna. Because RS-232 is single-ended, this noise and interference is impossible to distinguish from a real signal. Every(?) protocol designed for longer distances or faster speed will use differential signaling, which, roughly speaking, sends a mirrored copy of the data at the same time as the normal version. That way, the interference can be canceled out.
The closely related RS-422 and RS-485 will work far better, and it is easy to convert from RS-232 to RS-422/485 and then back to RS-232 on the other end. These standards were designed to run over the lengths of cable you want, but has the small drawback of requiring twice as many wires for the data. RS-485 can work in half-duplex mode, using the RS-232 RX/TX as one pair, if you don't have enough signal lines.
Numerous converters exist on eBay for 2 USD with free shipping, or, depending on your application, there are TTL<->RS-422/485 converters for microcontrollers.