atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services T1's, CSU's and DSU's 56K/DS0 loopback plug

It has been quite some time since I've had to turn-up and test DS0's, but I am needing a loopback plug to test a 56K circuit. I have my circuit terminated on a RJ48S, using R1 on pin 1, T1 on pin 2, T on ping 7, and R on pin 8. I was reading on the forums that the loopback connects pins 1-7 and 2-8. Shouldn't the proper loopback on this actually be 1-8 and 2-7?

Polarity is NOT important for metallic loopback tests, so it's a "Don't Care". Simply turning around the bits given to you is enough for that level of testing.

Same goes with T1 circuits...same goes with Ethernet.

Nope...

I’m going to guess you’re probably confusing what the “T-s” and the “R-s” are designating… You’ll see T, R, and R, R1 used to notate the pin-outs. Easy enough to confuse those for “transmit” and “receive” but those names signify Tip, Tip1, Ring, Ring1. A loop-back plug would connect Ring/Tip (the first pair) to Tip1/Ring1 (the second pair.)

So you loop the Ring (R) side of the first pair (pin 8) to the tip side (T1) of the second pair (pin 2); and Tip side of the first pair (T - pin 7) to the Ring side of the second pair (R1 - pin 1)?

Makes more sense to me to put ring to ring and tip to tip....but I will go w/ what you are saying, you are the experts! Thanks for the info.

Pin 1 = R1
Pin 2 = T1

Pin 7 = T
Pin 8 = R

Jumper pin 1 to pin 7
Jumper pin 2 to pin 8


Each pair works together as a transmit path or a receive path. A loop-back plug turns one PAIR back to the other. Gives the 1’s and 0’s a round trip ride. A loop-back.

Connecting tip to tip1 and ring to ring1 would create a short at the end of each pair. A pair could be tested that way but would be “dead-end” for any transmission tests.

Thanks for the help and explanation. It is appreciated.

Grab an RJ45 connector and loop pins 1-7 & 2-8 and have at it