Executive Editor
From an email chain among a group of technical editors, the following questions recently arose: Why does the idea of mounting electronics in a 19-inch rack persist? Further, why was 19 inches chosen in the first place?
As I point out in a recent blog post, Practical-Home-Theater-Guide.com traces the 19-inch rack back to George Westinghouse in 1890, who used 19-inch shelving to house railroad relay gear. The site notes that telephone companies eventually adopted the 19-inch width, and the EIA standardized the 19-inch structure in 1965 with its 310-D standard.
According to Wikipedia (under the entry “19-inch rack”), a revised EIA standard in 1992 gave us today’s 1.752-inch (44.5-mm) 1U height.
Responses to my blog post offer several opinions. Brainstorms writes, “Ultimately, all dimensions are arbitrary. That they correspond to a nice, round number [or prime number in the case of the 19-inch rack] in some equally arbitrary measurement system is coincidental (whether intended or not). Of course, those round numbers fascinate human beings. But what’s really of importance is that the dimensions be standardized and that those standards be adhered to by various manufacturers.”
Steve Alonso comments of the 19-inch dimension: “It ranks up there with the 4-foot 8.5-inch rail spacing based on the standard Roman chariot/cart width conforming to the width of two horses side-by-side.” The myth-busting website Snopes.com questions whether the rail gauge actually descended from Roman times, calling the claim that it did “… partly true, but for trivial and unremarkable reasons.” In any event, Alonso is certainly correct when he concludes, “We are all victims of historical choices for better or worse.”
Sam Simione writes, “… the 19-inch rack-mounting size is manageable and works well from the viewpoint of a practical size we can build, handle, mount, and ship. Also, 19-inch rack enclosures are built such that they are relatively mechanically stable even at 40 and 45U heights.” William Ketel agrees that a 19-inch assembly would be easy for one person to grab with both hands, adding that the width might be a multiple of the widths of the relays Westinghouse wanted to mount as well as a submultiple of a standard width of sheet metal available at the time.
Simione adds that anyone believing the size is not optimum for modern electronics should bring the issue to the attention of the EIA.
In fact, there seems to be no need for revision. As Wikipedia notes, “The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably, and the set of fields to which racks are applied has greatly expanded.”
That’s for sure. The latest example of the technology that fits within the format comes from National Instruments, with the introduction last month of its new PXIe-5840 vector signal transceiver, which combines a 6.5-GHz RF vector signal generator, a 6.5-GHz vector signal analyzer, a user-programmable FPGA, and high-speed serial and parallel digital interfaces.
Charles Schroeder, vice president of product marketing for RF at NI, said in a recent interview that NI engineers worked hard to fit the new VST’s functionality into a two-slot PXI Express module. (The original VST, introduced in 2012, required three slots.) They succeeded, enabling eight VST modules to fit it in a single 18-slot, 19-inch chassis, forming an 8×8 MIMO test setup.
As Simione put it, “Whatever the reason for the 19-inch dimension, history has proved it was a good choice and is here to stay!”
