On the South Birmingham LUG list this morning, someone asked a really interesting question:
Why are servers (and other rack mounted equipmnent) measured in U and why is 1U 1.75 inches? Obviously there has to be some sort of standardised measure, but why 1.75 inches? Why not 2 inches or 5cm or 1 hand? And why are racks 19 inches wide? Why not 20inches?
I remember hearing a similar question of why railway tracks (the gauge) are the size they are. I've heard the answer on several occasions, but can't remember at the moment. But I've never thought about rack sizes before. Anyone know why, or have any links to explain them?
Do you mean the old story about standard gauge being based on the width of Roman chariots? It's a nice story, but Snopes and Wikipedia both have it down as an urban legend.
Re:Standard Gauge
barbie on 2005-05-18T12:58:59
I don't recall the story about Roman chariots. There were several different gauges originally, depending upon which part of the country you were from. Most of the original engineers worked for mining companies, where most of the original steam engines (not rail based) were thought up and used.Incidentally one of James Watt's steam pump engines can be seen on the A38 Aston Expressway on the way into Birmingham.
Re:GPO
merlyn on 2005-05-18T11:49:56
Isn't it the "submarine hatch" requirement? As in, the equipment has to be able to fit down through a "standard" submarine hatch, and that's something like 20 inches.Re:GPO
n1vux on 2005-05-18T17:56:35
Re Submarine hatches, The "standard 25 inch diameter submarine hatch" is a constraint on the diameter or hypontenuse of the frame, not directly on the width. I did work with a vaguely VAX-like Sperry mini-computer that was offered in "standard" and "octangonal" racks, the latter to slide down (or up) a submarine hatch. I think it was the AN/UYK-43, not it's little brother the AN/UYK-44. I gather it had it's own maintenance difficulties
:-). The 19-inch Rack history is available from the usual source, which agrees they come from the telephony relay racks, but doesn't specify further.
The Usenet has deeper memory
... 19" is in some sense a misnomer, as the interior dimension is 18"+-. The 1.75 inch U-Unit is apparently either an RCA or a Western Electric Co (WECo) heritage (WECo had a 23 inch spacing). Whether the RCA 19in measure was influenced by the British GPO, it doesn't say.
