APLawrence.com -  Resources for Unix and Linux Systems, Bloggers and the self-employed

Tony Lawrence: RS232 (The Lights are On) (1991)

© January 1991 Tony Lawrence


The bane of my life is RS-232 communication. This one area has caused more wringing of hands and tearing of hair than anything else. And while just plain RS-232 to RS-232 hookups are bad enough, the addition of modems makes things worse. If the modems are high speed, data compressing models, I start quivering before the cables are even hooked up.

There was a time when I thought that my problems could be alleviated by an increase in knowledge. I felt that I really didn't know enough about RS-232 in general and modems in particular, and that this knowledge gap was contributing to my confusion and despair. It seemed logical to assume that a liberal application of books, tools and playing around could only bring good results.

Well, I have the books, and have read all about MNP and V.42bis. I have puzzled over Phase Shift Keying and Quadrature Amplitude Modulation. I've bought Gender Changers and Null Modems and Breakout Boxes, along with a horribly expensive crimper that attaches modular plugs to phone wire cables. I have played with cables and modems and software of all descriptions. In short, I have given myself a short, intensive course in the theory and practice of RS-232 communication.

Certainly these efforts have improved my understanding of what is supposed to happen when two computers reach out and touch each other with their DB-25's. Unfortunately, what is supposed to happen is generally not applicable to life in the real world. I still find myself typing AT commands into stubbornly unresponsive modems or watching garbled and unreadable messages scroll up and down totally confused screens.

One of the most frustrating situations is when two modems have been carefully matched, lovingly configured and quite properly introduced to each other. They have accomplished their ritual handshaking, exchanged squeals of delight, and have settled down with all the proper lights aglow, seemingly throbbing in anticipation of the joyous exchange of data that is to come. They then refuse to transmit or receive even one single character. Frantic conversations with my counter-part at the other end ensue. "Yeah, my 9600 light is on- is yours?" "My keyboard is dead- no, I think the whole system is dead!". Breakout boxes with festive red and green leds are applied to no avail. Port configurations are checked and re-checked. Accusations of subscribing to sub-standard long distance services are invariably bandied about, and when things get truly desperate, the very integrity of the Operating System is called into question.

In such situations, the modem happily retains the connection with it's mute friend, while the two of them gobble up Message Units. Guard signals (+++), which are supposed to summon the modem to attend to commands, are blithely ignored. Nothing short of unplugging the phone is likely to convince either modem that the intended mating has not worked. The occasional blinking of a Data light only adds to the mystery: if these two devices are talking, what on earth are they saying?

I know exactly what they are saying: "It's that Lawrence guy again- and we are REALLY driving him nuts this time!"

Got something to add? Send me email.

(OLDER)    <- More Stuff -> (NEWER)    (NEWEST)   

Printer Friendly Version

-> RS232 (The Modem Lights are On)

Inexpensive and informative Apple related e-books:

Take control of Apple TV, Second Edition

Take Control of iCloud

Take Control of Apple Mail, Third Edition

Photos for Mac: A Take Control Crash Course

Take Control of Preview

More Articles by © Tony Lawrence

Printer Friendly Version

Have you tried Searching this site?

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.

Contact us

Printer Friendly Version

Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it? (Brian Kernighan)

Linux posts

Troubleshooting posts

This post tagged:



Unix/Linux Consultants

Skills Tests

Unix/Linux Book Reviews

My Unix/Linux Troubleshooting Book

This site runs on Linode