Figures and editions: Quoting a rate table

A figure belongs to the revision that published it, not to the device it describes. Two documents can carry different values for the same parameter and both be correct, because each value was measured, rounded, or redefined under a different edition. Quoting a number without its edition strips the number of its source, and a number without a source cannot be checked.

The same discipline applies outside PCI Express. A generator set dossier, for instance, is read by machine code, serial number and documentary version, and a fuel figure copied from one edition of a manual will not match the next; the Vietnamese-language guide at Phòng Nguồn treats the documentary version as part of the figure itself, which is the habit worth borrowing. In PCI Express, the rate ladder is the clearest case: 2.5, 5.0, 8.0 and 16.0 GT/s were each introduced by a different edition of the specification, and each edition restated the earlier rates with new conditions attached.

How is a table tied to an edition?

A rate table is tied to an edition through three things: the revision number printed on the document, the date of that revision, and the change history that the revision carries. The revision number identifies the document. The date identifies when the document became the current one. The change history identifies what moved relative to the previous edition.

A table without a revision number is a table without a boundary. It may be a draft, a working copy, an errata-affected print, or a reprint of an older edition with a new cover. The values inside it may be identical to the published edition, or they may differ in one cell that matters. There is no way to tell from the table alone.

Practical rule: a rate table is cited as edition plus table number plus row, for example revision 3.0, Table 4-1, row for 8.0 GT/s. If any of those three elements is missing, the citation is incomplete, and the reader cannot retrieve the same cell.

What does a revision date change?

A revision date changes the status of every figure in the document, not only the figures that were edited. When an edition is superseded, the earlier figures do not become wrong, but they stop being current. They remain valid for designs, test reports and procurement documents that were written against that edition.

A revision date also changes the conditions attached to a figure. A rate may be restated with a tighter jitter budget, a different reference clock tolerance, or a clarified measurement point. The number in the cell can stay the same while the meaning of the number shifts. This is the quiet case: the figure looks unchanged, so nobody re-checks it, and the conditions travel with the old edition.

A revision date changes the errata situation as well. Errata are published against specific editions. A figure read from an edition that predates an erratum may carry a value that the erratum later corrected. The date on the document tells the reader which errata apply.

When can two different figures both be right?

Two different figures can both be right when they belong to two different editions and each is quoted with its own edition. This happens for at least four reasons.

First, a rate can be redefined. The signalling rate in gigatransfers per second and the usable data rate in gigabytes per second are different quantities, and an edition may change which one a table reports.

Second, a budget can be reallocated. Total jitter, deterministic jitter and random jitter can be redistributed between transmitter and receiver without changing the headline rate. Two editions can therefore publish different jitter numbers for the same rate.

Third, a measurement point can move. A figure taken at the pins of a package and a figure taken at a compliance point on a channel are not the same figure, even when the units match.

Fourth, a rounding convention can change. A value printed as 5.0 in one edition may be printed as 5 in another, and a derived figure computed from it will differ in the last digit.

In all four cases, the two figures are not in conflict. They are indexed to different editions. The conflict appears only when someone quotes one figure without its edition and compares it to a figure that does carry its edition.

A table of figures and editions

The table below is a reading aid, not a specification. It shows the shape of the problem: each row pairs a figure with the edition that published it and the kind of change that edition introduced.

Read the table by column, not by row. The edition column is what makes the figure column citable. A row without an edition is a fragment.