J.P. Larocque (jp_larocque) wrote,
J.P. Larocque

Yost serial wiring

Warning: only the most pathologically geeky need continue.

Today I got my serial situation sorted out.

I use RS-232 (serial ports) for machine management on an out-of-band medium. If the network is unavailable and I can stick a serial cable into some machine, I can log into it, poke around, fix whatever needs to be fixed, and watch most bootup messages (after POST, beginning with the bootloader), all without needing a video signal or a keyboard. (Expensive motherboards and expensive add-on cards will also capture BIOS-printed text to a serial port; of course Suns and serial-equipped NewWorld Macs do this as a matter of course. I think the most Enterprisey versions of Windows support a serial console, but I don't have any experience with that.)

That's handy, because video only goes so far. If I have a serial cable between machine X and machine Y already, I can log into and monitor machine Y by SSHing into machine X, nomatter where I am physically.

It's a little known secret that serial cables you'll find at RadioShack or Best Buy are outrageously overpriced. Actually, it's a well-known fact. Today I received my order of RJ45 to DB-9 and RJ45 to DB-25 adapters, 34 in all. That plus the abundance of free cable I'm sitting on and admittedly more time than I was expecting adds up to hundreds of dollars worth of serial cable. At whatever arbitrary lengths I need. The whole thing came to under $25 shipped from monoprice.com.

Here's how it works:

  • First I wired the adapters I'll be using. These take an RJ45 plug (like Ethernet) and give you some sort of DB-9 or DB-25 connector (like most serial ports use).

    The wiring is fixed per device, and considered a semi-permanent fixture. Therefore, I screw each of the adapters into each port that they'll live on for the foreseeable future. Even if I'm not using that port now; even if I change which other device a cable on that port runs to, the adapter never changes.

  • Then I cut off lengths of 8-conductor cable. It can be just about anything; I have mostly Cat-5 lying around (as do most people (that have read this far)), but I also happen to have some 4-line telephone cable, which is non-twisted and therefore useless for even 10BaseT. Perfect for this.

    The pinouts I use for the cables are always the same for this system.

  • Voilà! I plug any cable in-between any two devices with these adapters hanging off of them. (It turns out the tabs on RJ45 connectors are exponentially more convenient than the thumbscrews of DB-9 and DB-25.)

Did you notice how I didn't mention anything about null-modem cables? Under this scheme, every cable is a null-modem cable, and every adapter is wired for the kind of device it connects to, and expects this kind of cross-over/null-modem cable. So servers and terminals are wired one way; modems are wired another. That's my favorite part about the scheme. As long as you stick to it, you don't have to worry about whether a particular cable is a null-modem cable. It's quite elegant.

What is the scheme? It's the "Yost Serial Device Wiring Standard," 21 years old now. Anyone that's doing anything with serial needs to be using this. It's loads more convenient and cheaper than ready-made serial cables.

So, instead of scrounging for one of the two serial cables I own, I've now got five cables permanently in-place. One of them is 25 feet long, strung over a doorway, instead of 10 feet long, with a bulky DB-25 null-modem in the middle connecting two segments and strewn about the floor, ready to be tripped over. I can SSH into my most trusted system to access the console of any other in my closet.

And, I've now got a UPS cable. Apparently APC considers their cables business (WTF?) to be Serious Business, and protects their trade secrets (pinouts) vigilantly.


I found this diagram from the makers of my UPS software.

Following this pinout, I was pleased to find that I can make Yost DB-9 M adapters for my APC UPSes and use the same serial cables with them. Assuming standard coloring (which is also the coloring as is used in the Yost standard), cut off the pins for brown, orange, blue, and white. Solder the brown and orange leads together, and (separately) blue and white. Cut off green's pin and solder it onto red's pin (or vice-versa) as you would any other Yost adapter. Then push the black pin into position 1 on the DB-9 connector, the yellow pin into position 2, and the red/green pin into position 9. There's your $30 cable.

Needless to say, the biggest downside of going this route was that it took me about 4 hours to wire just 23 adapters. But that's a one-time cost; making new cables (as I need them) takes just a few minutes.

The next-biggest downside was that the adapters I purchased had the pins pre-crimped on the leads. Which was helpful, considering I don't have the pins or crimper for these adapters, until I realized that for each adapter, I'd have to cut off the pin of one wire, strip that wire, and solder it onto the pin of another wire: signal ground, which is green and red on the RJ-45 side, connects to just one pin on the DB-9/DB-25 side. That's what took most of the time. And mashing those now-oversized pins into the connectors.

But I'm still pleased, and glad to get this years-long todo item crossed off.

Tags: geeky, hardware, serial
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