Long distance Communicating between PICs

[koossa]

- What is the maximum distance that I would get with RS232?

- Will it be possible to connect a 12V transformer on 2 of the wires and at every station have a 7805 chip to take it down to 5v?


Regards
Koossa

[mackrackit]

The best I been able to do with any reliability is about 85 feet using RS232 and one of the MAX inverter chips. The extra chip is to generate true RS232 voltages in this case.

The standard for RS232 will swing from -12volt to +12 volt. The PICs running on 5 volts work fine as do most of the computers that have a serial port with a +/-5 volt swing(have not seem a true voltage level for some time) for maybe 50 feet with good cable.

The 7805 would also work to power the nodes.

[Ioannis]

On what cabling to connect the transformer? On RS-232 standard cabling there are signals on every one of the 9 cables. So this is a little risky.

A 12V could do OK I supppose on short length. But on the 7805 a heat sink should also be used, and depending on the load, maybe a large one. That's why I suggest a cheap and easy switch mode regulator.

Ioannis

[koossa]

I will be using the RS485 Interface and just want to make sure that I'm on the right track (See Attachment)?

I want to make use of 4 twisted pair cables, 2 for the Serial comms between the stations and 2 for the power (don't know if 12volts is too much?)

The Base station (Top) will continually ask each station it's status. When it receives a reply or reach a timeout it will move on to the next one.

The cable length between the stations will be about 20 meter (65ft) and I rather want a reliable connection than a fast one.


Thank you
Koossa

[Ioannis]

That exactly was the idea that had implemented long ago using standard UTP cable and distributing power through the free pairs to the slave devices. My setup had 1 master and up to 4 slaves at very slow speeds. But when all 4 slaves were connected, one was loosing communication from time to time. I have not found why up to this moment.

I am suspecting that the bus is at 3-state when no device is active and this is producing an error.

I used the ADM483EAR chip instead of SN75176 because of the fail safe protection. On a recent system I returned to the good old 176 and everything is cool. I added external resistor for the fail safe protection though.

Ioannis

[koossa]

Thank you very much Ioannis!