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The 3B2 computers have eight-pin RJ45 connectors, and many of us are faced with trying to talk to them via our standard PC serial ports. It's possible to make this work with a combination of standard cables and adapters, but it's frustrating as hell sometimes. So I have built my own RJ45 to DE9 adapter to make it much simpler.

For some reason, the 3B2 family uses "flipped" cables, which join pins 1/8 2/7 3/6 4/5. We have completely abandoned this whole idea in favor of "straight through" cables, which means we can use more or less any Ethernet cable that runs all eight pins. We suppose we should not be ignoring the little ground clip, but at this point we're a little on the lazy side.

So: we adopt a pin-one-on-the-right-of-the-jack paradigm for all our connectors, so when looking into the RJ45-to-DE9 modular adapter, pin one is as shown. This is explicitly backwards from the AT&T console, PORTS and EPORTS pinouts.

[RJ45 Connector]

DE9 female                    RJ45
connector                   connector

DCD 1 <----+--------------- 5 DTR (Data Terminal Ready)
DSR 6 <--'

RD  2 <-------------------- 6 TD  (Transmit Data)

TD  3 --------------------> 4 RD  (Receive Data)

DTR 4 --------------------> 3 DCD (Data Carrier Detect)

SG  5 --------------------- 2 SG  (Signal Ground)

RTS 7 --------------------> 1 CTS (Clear To Send) - usually blue

CTS 8 <-------------------- 7 RTS (Request To Send)

RI  9 <--  n/c       n/c -- 8 PG  (Protective Ground)

The CTS and RTS signal lines are not used by the console or PORTS serial lines, but they are used by the EPORTS. We prefer to make one single kind of connector for all possible uses. Furthermore, in a pinch a simpler connector by omitting the jumper to the DE9 pin 6 (DSR), so the RJ45 would route pin 5 (DTR) to pin 1 (DCD) directly.

Remember to use a straight through cable!

Note: the 9-pin connector is correctly a "DE9", not a "DB9". The "DB" designation is for the larger 25-pin connector.