Cable Diag

[tomtech]

Not sure if this is a "problem" or something else, but i noticed that the cable diagnostics have begun reporting shorts on several of our connected devices. we have a tower with a WS-12-250-AC and 6 connected UBNT devices. one showed shorst on two pairs. We powered down, tested the cable but our tester showed all good. we replaced ends on the cable and still tested good with outboard gear but the switch reported two open pairs. we changed ports on the switch and got the same result. i changed the Radio and still have the same result. then i tested one of the other sectors and got the same two open pairs.
everything is working OK, but every 100mb radio with 24v is showing pairs 3 and 4 shorted.

Let me know if you need other info.

Thanks

T

[sirhc]

You will always see shorts on pair 3 and 4 of 10/100 UBNT devices and many other 10/100 equipment.

Here is a good idea, in your LAB play with different devices and see what each device reports that way you know what to expect in the field and will be able to spott things that do not look right.

Also this has been discussed many times, and there are some posts that everyone should ready like these below (read the whole post):

viewtopic.php?f=6&t=1178

Question: Why do I see short on pairs 3 and 4 on a of devices especially UBNT airMAX 10/100 gear.

Answer: Because every manufacturer does not follow the proper design specs and use a 4 channel Ethernet Transformer designed for POE with a center tab for each pair. Since pair 3 and 4 is not needed to carry data with 10/100 only links they use them for power only which allows them to use a cheaper 2 channel transformer without a center tap for pairs 1 and 2. They then connect the 2 wires from each pair to the board skipping the need for a transformer with a center tap so the 2 wires from pair 3 are connected directly to the board shorted together for positive and the 2 wires from pair 4 are shorted together directly on the board for negative.

viewtopic.php?f=6&t=1259
viewtopic.php?f=6&t=1215
viewtopic.php?f=6&t=1549
viewtopic.php?f=6&t=1037

Here are some good posts on grounding:
viewtopic.php?f=30&t=1816
viewtopic.php?f=30&t=188
viewtopic.php?f=30&t=1429
viewtopic.php?f=17&t=1786&start=30#p13447
https://community.ubnt.com/t5/airFiber/ ... rue#M31070

Grounding was the #1 thing I learned that saved me so much equipment, time, and customers.
GROUND IS NOT GROUND