Wednesday, February 1, 2012


Baud certainly signals a four letter word is pending.

Last weekend, my work took me to a machine room to visit some of the machines I help maintain. This is a pretty rare activity for me - usually the hardware I work with is locked away in a server closet or the equivalent, but occasionally I need to visit the equipment to perform some maintenance where it is best to have a person on-site in case of problems. Usually the problems, if they appear, are easy to take care of, but some of the servers were getting a little past their prime, and we were concerned what would happen if there were serious problems.

Most of the work went fine. Some surprises that did require a person on-site, but nothing terribly bad. We were down to our last two machines - some very old boxes from Sun Microsystems that we couldn't attach to a keyboard or monitor, because they used non-standard connections.  After rebooting the machines... they didn't seem to start back up. Now we had a little bit of a problem, since there was no good way to connect to them to find out what happened. Their Internet connections weren't responding, we had no keyboard or monitor on them, so the only hope we had to connect to them were through non-standard serial cables.

Serial cables have been used for decades when it comes to connecting two computer systems together. Terminals used to be connected to their mainframe systems via serial cables, and it was entirely common as recently as ten years ago to connect a modem to a computer using a serial cable. The USB cables we use today are just the most recent version of this innovation.

Unfortunately, for us, these machines far predated USB. A trip to Radio Shack didn't get us all the parts we needed, so we scrambled around the machine room and far-flung closets to see if we could put together all the parts we needed. The biggest problem with serial cables was that they were notorious for being finicky - not only did you need to find the right size cable connections, but you needed to make sure you got cables that could plug into each other, and then possibly toss in a "null modem" cable. And if it didn't work... you never knew if it was because you did something wrong with the cable or with the settings on the computer or if it was the wrong phase of the moon.

But we had done this before. We knew what we were doing, and we knew where the possible problems could be. Heck, we even knew the difference between bits per second and baud - concepts that most people get confused about.  Baud refers to the number of signaling state changes that are made on the hardware, dates back to the early 1900's to refer to telegraph equipment, and is named after J.M.E. Baudot, inventor of the teleprinter. With serial lines and modems, this was equivalent to the number of bits per second that could be transmitted over the line. In later generations of modems, however, compression was employed to get more bits into the same number of signaling state changes, so while the baud of modems did not increase, the number of bits per second did.

Amazingly enough, the first cable worked correctly on the first try, and we quickly discovered the problem. A typo in the reboot command left it waiting for additional information. There was some confusion with the port configuration for the second machine (there were three serial ports - all unlabeled), but in short order we got that machine running the same way.

We left the cables in place... just in case.

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