RS485

RS485 is a serial communications protocol that is very flexible; I describe only what I am interested in using here. It can be full duplex, half duplex, bus, point to point...

I am looking for a way to wire up sensors and controllers in my house, and I am doing it with wire. Wireless is great but I find wire to be more reliable. I have already run wire for ethernet and radios and speakers... running wire for RS485 should not be too hard.

I will be using a half duplex bus topology with one master controller and probably 3 or 4 slaves. (A weather station, an AC power monitor, and a well pump house monitor/controller are the initial applications that I am investigating. Maybe more later.)

RS485 can do 10 Mbps for short links, but speed goes down with link length. 250 kbps is probably a more realistic ceiling. Since I plan on using PIC based controllers and moving very little data I will probably settle on something around 9600-38400 bps.

There is no data protocol defined in the spec. Roll your own or borrow something.

Uses differential signaling, single polarity. A "1" is +5 on the A line and +0V on the B line.

Usually uses twisted pair, but it wants a separate ground line so one pair for signal plus power and ground plus a shield would be good. I will probably end up using CAT5E cable (unshielded).

Interface chips

Texas Instruments

75176B - The old standard

75LTC184 - Drop in replacement for the '176. New and improved.

Terminator and bias resistors make this a bit of a pain to work with.

Maxim

MAX481, MAX483, MAX485, etc

These chips seem to be the way to go, I will be getting some to test.

Computer connection

RS232 to RS485

This is what I have been testing, built an adapter with a MAX202 and a 75176B. I can buy a similar adapter on Ebay for $7.00

My laptop has no RS232 and my server has only one serial port which I want to use for the Magnavox GPS reference station.

USB to RS485

USB is pretty easy to deal with and my server has 6 USB ports. Rolling my own adapter seems like a waste of time.

There is one that is open frame and has an FTDI chip, a MAX485, and a MAX202 so it does USB to serial TTL, RS485, or RS232 all on one board. The Maxim chips are socketed. I like the fact that I can see all the chips in the photos so it is almost as good as open source hardware; I know what I am getting. I paid $17 including shipping. (09-Jan-2009)

$10-18 on Ebay

PCI cards

A 2 port RS485 PCI card from Ebay (shipped from Hong Kong) is less than $10. This would be great but my server has only one slot and it has a NIC in it right now.