RS485

Very flexible; I describe only what I am interested in using here.

Half duplex multidrop bus topology with one master controller and up to hundreds of slaves. Master polls slaves.

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 like 9600 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.

$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.