Re: possible distance for the microcontroller output signal

evaradharaj · Joined: 15 Jan 2009 Posts: 60

Hi,

I am using PIC 16f877a. I need to send the microcontroller output signal from portB7 and portB6 to the longer distance about 10 meters.. Is it possible to send it through wires? I am planning to use coaxial cable...!

There is no possiblity to use wireless.. so that i am choosing wires..

Thanks.
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FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Coaxial cable isn't bad, but a differential signal over twisted pair, using RS422/RS485 drivers and receivers would offer higher noise margin. For the single ended (coaxial cable) voltage, a higher voltage, e.g. driven by a MAX232 would be preferred.

A minimal solution would be to place a series resistor (e.g 100- 200 ohm) to isolate the cable capacitance from uP IO pin. Overvoltage protection may be suggested according to th eenviromental condition.

temtronic · Posted: Tue Aug 23, 2011 7:21 am

I second the use of a MAX232 chip ! GREAT to debug with as you can send data to a PC to confirm 'it works' the way you want. The MAX232 will give you 2 complete serial ports.One to the other PIC the other just for testing or 'future' use.The cost for chips and board space is nothing compared to wasted time trying to get the 'bugs' out without visual feedback of what's going on.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

What kind of signal are you sending? What frequency, slew rate, current level, etc.. If it is 1200 baud serial don't worry. If it is 115200 baud or if it is a high speed camera strobe things could get dicy. You may also need static protection if there is a long cable that will be handled a lot. If there is other electrical machinery around you may need shielding. If a lot of different cables may be used you may want protection from a mis-wired cable putting odd voltages on the PIC outputs. Also consider the consequences if a signal or two is lost. It may be crucial, or it may be no big deal.

MAX232 chips are good for a lot more than just serial communications signals. They provide protection for the PIC, a decent line driver, and reasonable noise rejection if you need it.

Distance alone is not a problem. I send bidirectional 300 baud serial over 1000 meters between two PICs underwater by simply capacitor coupling (to stop +24V from a mis-wired or shorted cable) from each PIC to the twisted pair wire, and each PIC has as resistor to assert Stop bit when idle. It has worked fine all over the world for over 12 Years.
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