Re: XAUI IO specs
All,
The past can come alive. For the recent discussion of
Vdif vs Vdif peak to peak,
Howard Johnson had a wonderfull and crisp definition of these terms for 802.3z when the same subject came up.
His e-mail is reproduced below, as I can not improve upon it.
Tom Mathey
A reflector lurker
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Date: Tue, 4 Mar 97 14:10:30
To: stds-802-3-hssg@xxxxxxxx
From: Howard Johnson <howiej@xxxxxxxxxx>
Subject: Peak-to-peak Differential Voltage
Regarding the recent flurry of messages about the differential amplitude specification for clause 39 (short copper), I would like to point out that the existing specification may be met with a differential output (that's two signals) each of which has a 550-mV logic swing. That's right. Only 550 mV is required.
The picture below depicts a typical circumstance, involving two complementary, wimpy, PECL-like drivers. (a fixed-width font, like Courier, is required to display this picture)
-(A)---- ---------------------------- (3.75 volts, logic HI)
\ / |
\ / |
\ / |
/ (+550 mV peak differential amplitude)
/ \ |
/ \ |
/ \ |
-(B)---- ----------------------------- (3.20 volts, logic LO)
(1) Each signal has a peak-to-peak range of 550 mV (3.20 to 3.75)
(2) The differential signal (A-B) ranges from +550 mV to -550 mV
(3) The PEAK differential signal is 550 mV
(4) The PEAK-TO-PEAK differential signal is 1100 mV (this meets the minimum requirement in clause 39).
The term PEAK-TO-PEAK always means the difference between the most positive and most negative readings of a particular signal. In this case we have: (+550 mV - (-550 mV)) = 1100 mV. To say that you cannot meet the clause 39 specification is to say that your individual outputs cannot each source 550 mV peak-to-peak. It would be *very* surprising to me to hear that any CMOS vendor could not meet this specification. In conversations with many individuals this morning I have determined that, as Bob Rumer suspected, many individuals have mis-interpreted the phrase "peak-to-peak differential amplitude". In preparing for a possible re-wording, I have dug out the following references from other 802.3 clauses.
NOTE: in 802.3 10BASE-T, 14.3.1.2.1, the standard defines the PEAK differential amplitude (not PEAK-TO-PEAK).
NOTE: in 802.3 100BASE-T4, 23.5.1.2.1, the standard defines the PEAK differential amplitude (not PEAK-TO-PEAK).
NOTE: in 802.3 100BASE-TX, we reference an ANSI standard, ANSI X3.223 (FDDI TP-PMD clause 9.1.2.2), which defines the PEAK differential amplitude (not PEAK-TO-PEAK).
NOTE: in 802.3 100BASE-T2, 32.6.1.2.2, the standard defines the PEAK differential amplitude (not PEAK-TO-PEAK).
Perhaps, since we are writing an 802.3 standard (not a Fibre Channel document), we should convert table 39-1 to use units of PEAK differential amplitude, instead of PEAK-TO-PEAK. Relavant entries for table 39-1 might then read:
------------------------------------------------
Peak differential amplitude (MAX) (NOTE a) 1000 mV
Peak differential amplitude (MIN) (NOTE b) 550 mV
NOTE a: The magnitude of the differential signal may at no time exceed this value.
NOTE b: The magnitude of the differential signal between normalized bit positions 0.29 and 0.71 (see figure 39.3) may at no time fall below this value.
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