Anybody remember Token Ring ? The pin assignments, the split 3 & 6 pins, is so the same cable and connectors can be used with both ethernet and token ring. The RJ connectors were picked because they were readily available in large quantities, and the Bell definition made them quite reliable for multiple insertions.
Doug Smith <d...@emcesd.com> wrote: Hi Doug and the Group, I believe the RJ-45 pin assignment had more do to with preventing an Ethernet interface from being damaged if plugged into a digital phone interface and possibly other interfaces. At that point in time, Ethernet was pretty slow and the strange pinout was not as much a issue as it is now. When I was working with the UTP (unshielded twisted pair) folks at Bell Labs (about 25% of my work for several years, I was shared between many projects as sort of an internal consultant) we proved time and again that UTP works as well and often better than STP (shielded twisted pair) for both operational performance and EMC issues. The presence of the shield in STP caused EMC problems because it was generally not properly connected in many designs and makes a great fat wire antenna and way to get noise into the system especially for PC interfaces. Bell Labs was a great place to work in the past. We had technology 10 to 20 years and more ago that I am just beginning to see in my client's equipment. In 1994 we demonstrated something very close to Gigabit Ethernet. I still have a four inch section of the 100 meter cable the demonstration was done over, embedded in plastic as a momento. An interesting observation: In one experiment I did, I measured the differential voltage on one of the pairs of a Cat5 cable that was being fed EFT in common mode using the Capacitive Clamp using one of my balanced coaxial probes ( http://emcesd.com/pdf/cd94scr.pdf - 3.5 MB pdf file) and noticed an effect that would affect EMC testing. When a Cat5 cable was bent sharply the balance got noticeably worst. When straightened out most but not all of the balance returned. The conclusion is that gently handled new Cat5 cables should be used for an EMC test. After that give them to the IT people and use new cables for the next test. By the way, the twist "wavelength" in a Cat5 cable of the pairs is different for different pairs to reduce crosstalk by making sure individual wires were not always next to each other during the same part of the twist going down the cable. Doug Powell, Doug wrote: > An interesting discussion, > > Of course, the 2-pair is for the RJ-11 connector and the 4-pair is for > the RJ-45. I was always under the impression that the choice in pairs > and colors was a result of a choice made at Bell Labs (part of the Bell > System). And this was a carry-over from the 4-pair cable that I saw > elsewhere in the Bell System. Also, this cable was physically > compatible with punch blocks found in telephone equipment rooms. > > One of the things I observed in the original 2-pair voice-grade wire was > the practice of using the yellow conductor as a telephone set ground. > Since digital networks needed a second data pair, the jump to the > already available 4-pair cable made sense, in order to keep a grounding > conductor available in the cable. Besides the Telco system was famous > for keeping spare pairs of wire handy, making service and upgrades > easier. > > I can't begin to imagine how much length of this cable has been made > over the years. So with the availability of this cable it may have been > a choice based in economics as much as anything else. What I cannot > begin explain is the seeming inconsistency in not pairing up the green > and white wires in the RJ-45 connector. > > RJ-11 > Pin color name > --- ----- ----- > 1 n.c. > 2 blk > 3 red ring > 4 grn tip > 5 yel often used for ground > 6 n.c. > > > RJ-45 > Pin color pair name > --- ----- ---- --------- > 1 wht/org 2 TX Data + > 2 org/wht 2 TX Data - > 3 wht/grn 3 Recv Data + > 4 blu/wht 1 > 5 wht/blu 1 > 6 grn/wht 3 Recv Data - > 7 wht/brn 4 > 8 brn/wht 4 > > Regards, > > -doug > > Douglas E. Powell > Staff Engineer > Corporate Compliance Department > > Advanced Energy Industries, Inc. > Fort Collins, Colorado USA 80525 -- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: d...@dsmith.org \ _ / ] \ _ / Website: http://www.dsmith.org - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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