h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Sitting here at my computer, I look up at my external modem and see all these pretty lights. But what do they mean? Is the evil tyrant Bill Gate (Personal opinion only) Communication with all the computer through out the world? Or are they from the electricity companies, getting us to use more power? Or are they there because the manufactures had a few spare LED's? We'll never know

Well... they're usually labelled:

RD = Recieved Data (from web to you)
TD = Transmitted Data (from you to web)
CD - Carrier Detect (Is there a working 'phone line?)
OH = Off Hook ("I'm using the line")
AA = Auto Answer (waiting for a call/fax)
HS = High Speed (using clever tricks...)
DTR = Data Terminal Ready (conversations between modem and computer)
MR = ??? (I have NO idea!)
PWR = Power

Hope that helps...

I asked around -
MR = Modem Ready
(more commonly called DSR or "Data Source Ready")

DSR = The "Data Set" (your modem) is online and available to the computer. It's not handshaking so much as a way to see if a modem is out there and powered up.

DTR = The "Data Terminal" (your computer) is available to the modem. The same bit of wisdom applies here in reverse -- the modem won't bother to anwser the phone if the computer isn't available. In fact, computers often drop this signal for a second or two two tell the modem to close a connection and hang up.

Mac modems actually use these lines to handshake instead of their original use, and I don't even think CTS/RTS are wired to the tiny RS-422 ports on Macs, so Macs and their modems interpret each other somewhat differently.