1A2 Converter: RJ-21 <-> RJ-45 Converter

These two cards can be used together to replace long runs of 25 pair cable in 1A2 phone systems between the KSU and phone extensions using runs of a few cat5 network cables.

Here's the pair of finished printed boards:

On the left, a "Key System" board which handles the KSU end of the connection, and on the right, a "Phone" board which handles the phone extension end of the connection. CAT5 cables connect the boards together, each carrying 8 1A2 signals. Up to three cables can be used, allowing a total of 24 1A2 signals.

Most 1A2 systems don't use even close to all the conductors in a 25 pair (50 conductor) cable, so shortcuts can be taken with CAT5 cables to carry just the needed signals. These boards are wired to provide common needs for a 1A2 system:

Cable Connector Provides Details
"A" Line #1 and Ringer A single CAT5 cable between the "A" RJ-45 connectors provides all necessary 1A2 signals for Line #1 and the ringer (bell). This cable provides T/R/L/A, Bell (Y-S pair), and two conductors for ground: total of 8 conductors. So with just one CAT5 wire you can hear ringing, pick up and answer calls on Line #1, put calls on hold, and dial out on Line #1.
"B" Line #2 and Line #3 Add this CAT5 cable if you want to access Lines #2 and #3. This cable provides the T/R/L/A and ground signals for Lines #2 and #3.
"C" Line #5 (ICM) and Buzzer Add this CAT5 cable if you want to access Line #5 (usually the Intercom) and the buzzer signals, which typically go together to faciliate intercom buzzing. This cable provides the T/R/L/A signals for Line #5 (ICM), as well as the buzzer conductors (Y-O pair).

You can provide these cables in any combination you need; from just 1 cable, to any combinations up to all three cables. Just be sure Cable "A" goes to the "A" connector at both ends; don't mix them up!

If you need more than what all 3 cables can provide, you might as well use a 25 pair cable, because more than 3 cat5 cables is already about as thick as a 25 pair cable. For any other kind of wiring, you can use "66 blocks" to punch down routing whatever signal paths you want.

So depending on what one needs, here are the common uses these boards were designed for:

PCB Layout Files

Here's the original ExpressPCB ".pcb" file I used for the board shown above, so you can print these boards yourself.

I later remade this into a Sprint PCB 6.0 .lay6 file which you can use to generate these gerber files which other print houses can probably make much cheaper, as ExpressPCB costs are kinda high for a board like this. Here's what the board layout looks like in Sprint PCB:

Note that the recommended Digikey part numbers for the three RJ-45 sockets and the separate male and female Amphenol connectors appear on the silkscreen, printed below the connectors. (e.g. "A31407-ND" below the RJ-45's, etc)

The two boards will be printed as one; you just saw down the middle with a jigsaw to separate the two boards.