Pile of random plugs and sockets


So many connectors – ever wonder how to hook something up?  First, there was the DB-25. Then the DB-9, Digital created the MMJ, someone else created the RJ-45, and so on. Meanwhile, the world moved its networks from coaxial cable to 10Base-T, which uses Cat 5 wire and RJ-45s (hey, Ethernet is serial, right?). There are ISDN and FDDI, too – don't assume that all RJ-45's are 10Base-T. Heck, ISDN even carries 90 volts, so be careful! In the following, "plug" and "male" are equivalent, as are "socket" and "female".

This document is a public service of Quayle Consulting.  This is copyrighted material.  Feel free to share as long as there's a backlink to https://quayleconsulting.com.

Quayle Consulting provides full evaluation and migration support. We have emulation solutions for these legacy hardware platforms:

Page Map

MMJ Adapters

The following is a list of common adapters from the DEC MMJ connector to various industry standard connectors. Wiring diagrams are available for these and other adapters commonly used on DEC/HP systems.

Are you in need of one of the following MMJ adapters? Quayle Consulting has a stock of these adapters available. Please contact us for a price quote!

MMJ to DB-25 female converter
MMJ to DB-9 female converter
MMJ to DB-25 male converter
MMJ to DB-25 male
MMJ to DB-25 male converter
MMJ to DB-9 female converter (works with standard PC serial port)
MMJ to DB-25 female converter
MMJ to DB-25 male converter
MMJ to DB-9 female converter
MMJ to RJ-11 male converter



Digital created the MMJ connector.  You can recognize it because the plastic clip is not in the center.  It was a great idea – the connections are simple, but no one else adopted the idea.

MMJ Pin Identification

The MMJ connector has 6 pins, numbered 1–6.

Socket – Looking into the socket with the clip down, the leftmost pin is pin 1.

Plug – Looking into the back (cable end) of the connector with the clip down, the leftmost pin is pin 1.

MMJ Pinouts

    1    DTR    (Data terminal ready)
    2    TX+    (Transmit data)
    3    TX-    (Transmit ground)
    4    RX-    (Receive ground)
    5    RX+    (Receive data)
    6    DSR    (Data set ready)

MMJ's use differential drive, just like RJ-45's.  Funny enough, the RJ-45 to MMJ cable is the hardest to build.

MMJ ends and crimpers are hard to find.  After hours of searching, I found that Ideal Industries sells them.  The crimper is product 30-497, and a bag of 100 MMJ ends for round cable (Cat 5 is round) is product 86-393.


The worldwide standard (sort of) is the DB-25.  Everyone calls this RS-232, but that name's been replaced by an EIA number that nobody uses.  Warning:  There are lots of pins in a DB-25 that are used for all sorts of obscure functions.  Don't assume that "no connection" means "no function".

Male connectors are usually used on PC's.  Female connectors are usually used on modems.  Usually.

Here's what the commonly used pins mean in a DB-25:

    1    GND    (Protective ground, almost never used)
    2    TXD    (Transmit data)
    3    RXD    (Receive data)
    4    RTS    (Request to send)
    5    CTS    (Cleared to send)
    6    DSR    (Data Set ready)
    7    GND    (signal ground)
    8    DCD    (Data carrier detect)
   20    DTR    (Data terminal ready)
   22    RI     (Ring indicator)


DB-9's are simpler.  They are not used for weird stuff (usually).  The DB-9 "standard" was developed by IBM because they wanted to save money – DB-25's are bigger, and so more expensive.  Here are the pinouts:

DB-9 Pin Identification

Note: Many DB09 connectors have numbers next to the pins on the back (cable) side.

The following diagrams show the connector from the back side.

      Female              Male
   -------------     -------------
   \ 1 2 3 4 5 /     \ 5 4 3 2 1 /
    \ 6 7 8 9 /       \ 9 8 7 6 /
     ---------         ---------

DB-9 Pinout

    1    DCD    (Data carrier detect)
    2    RXD    (Receive data)
    3    TXD    (Transmit data)
    4    DTR    (Data terminal ready)
    5    GND    (Signal ground)
    6    DSR    (Data set ready)
    7    RTS    (Request to send)
    8    CTS    (Cleared to send)
    9    RI     (Ring indicator)

Note:  The pinouts for early MicroVAX systems do not follow this scheme.  See the VMS FAQ for details.

Warning:  Token Ring networks also use DB-9's, but are definitely not serial!

RJ-45 (Serial)

Lots of equipment uses RJ-45's for serial connections.  They are cheaper than DB-25's and DB-9's, and are a lot easier to mount on a panel.  Here are the pinouts for the DECserver series of terminal servers.  DEC called this the "MJ8" connector.  Your mileage may vary with other devices.

RJ-45 Serial Pinouts

  1    RX-   (Receive ground)
  2    RX+   (Receive data)
  3    RX-   (Transmit ground)
  4    CTS   (Cleared to send)
  5    RTS   (Request to send)
  6    TX    (Transmit data)
  7    DTR   (Data terminal ready)
  8    DSR   (Data set ready) 

Note that the transmit and receive data are actually differential line drive, instead of voltages.  This is known as EIA-423.

Cisco Routers

Cisco routers have a different scheme (thanks to Jim Azbell):

  1    RTS   (Request to send)
  2    DTR   (Data terminal ready)
  3    TXD   (Transmit data)
  4    GND   (Signal ground)
  5    GND   (Signal ground)
  6    RXD   (Receive data)
  7    DSR   (Data set ready)
  8    CTS   (Cleared to send) 

Note that these lines are driven against ground, not differential like the DECservers. The Cisco documentation says that pins 1 and 8 are connected in their standard cable.

Flowpoint Routers

FlowPoint router console ports have yet another scheme (SpeedStream Routers):

  1    RX    (Receive data)
  2    RTS   (Request to send)
  3    N/C   (no connection)
  4    TX    (Transmit data)
  5    GND   (Ground)
  6    CTS   (Cleared to send)
  7    N/C   (no connection)
  8    N/C   (no connection)
Alcatel-Lucent Router 7750 with Cisco 3500 Switch

Here's an Alcatel-Lucent router 7750 with Cisco 3500 switch, provided by Sergio Antonio. I haven't tested this combination.

 RJ-45          DB-9
 1  RTS        8   CTS
 2  N/C        6   DSR
 3  TxD        2   RxD
 4  GND        5   GND
 5  GND        5   GND
 6  RxD        3   TxD
 7  N/C        4   DTR
 8  CTS        7   RTS

RJ-45 (Network)

RJ-45's are also used for network connections. The most common 10Base-T scheme is known as TIA/EIA T568B, which is:

RJ-45 Network Pinout

    Pin    Function
      1    Receive
      2    Receive
      3    Transmit
      4    unused
      5    unused
      6    Transmit
      7    unused
      8    unused

There are applications specified for ISDN, analog voice, IEEE 802.5/Token Ring, and other stuff.  I found a useful information in OPEN DECconnect System Overview, figure 2-2 and table 2-3.

DB-15 – Ethernet

DB-15 connectors are used as network connectors known as as AUI or MAU ports. Older systems referred to them as ThickWire Ethernet ports, since the old vampire taps used them.  They are still widely seen on equipment.

Transceiver modules are available to convert DB-15 to 10Base-T (RJ-45) networks. One such module is the CentreCOM 210TS.

DB-15 Pinout

The pinout provided here was published in a document that used to be on the Ericsson web site.

    Pin    Name    Function
      1            Shield
      2    ACOL+   Collision presence
      3    ATX+    Transmission
      4    GND     Ground
      5    ARX-    Reception
      6    GND     Power return
      7    CTL+    Control output
      8    GND     Ground
      9    ACOL-   Collision presence
     10    ATX-    Transmission
     11    GND     Ground
     12    ARX-    Reception
     13    +12VDC  Power
     14    GND     Ground
     15    CTL-    Control output

DB-15 Ethernet Loopback Pinout

The DEC Ethernet loopback connector 12-22196-02 has the following connections (from the DECstation 5000 hardware guide)

    Pins 3 and 5     Connected through a capacitor
    Pins 10 and 12   Connected through a capacitor
    Pins 13 and 6    Connected through a resistor and LED

AUI Transceiver Cables (BNE4C-x)

DEC made a series of cables, known as BNE4C, followed by the length in meters.  For example, the BNE4C-2 is 2 meters long.  It has a DB-15 male connector on one end, and a DB-15 female connector on the other.  Think about it being an "extension cord" for AUI.  The information I got off Amazon says:

AUI (Attaachment Unit Interface) Transceiver cable for connnecting MAU (Medium Attachement Units) to a MAC (Medi Access Control) commonly found on 10BASE-5 (thicknet) and 10BASE-2 (thinnet) networking systems.

    3    DO-A  Data Out Circuit A
    10   DO-B  Data Out Circuit B
    11   DO-S  Data Out Circuit Shield (not used)
    5    DI-A  Data In Circuit A
    12   DI-B  Data In Circuit B
    4    DI-S  Data In Circuit Shield
    7    CO-A  Control Out Circuit A (not used)
    15   CO-B  Control Out Circuit B (not used)
    8    CO-S  Control Out Circuit Shield (not used)
    2    CI-A  Control In Circuit A
    9    CI-B  Control In Circuit B
    1    CI-S  Control In Circuit Shield
    6    VC    Voltage Common
    13   VP    Voltage Plus (+12V)
    14   VS    Voltage Shield (not used)
   Shell PG    Protective Ground

DB-9 – Token-Ring

The AUI (MAU, ThickWire) DB-9 connectors have the following pinouts. (This is from a document that used to be on the Ericsson web site)

DB-9 Network Pinout

    Pin    Name        Function
      1    Receive-    Data in
      2                Frame ground
      5    Transmit-   Data out
      6    Receive+    Data in
      7                Frame ground
      8                Frame ground
      9    Transmit+   Data out

A loopback connector can be created with the following connections:

    Pins 1 and 5   Connected
    Pins 6 and 9   Connected

V.35 (X.25)

Another connector you might encounter is a 34-pin rectangular connector.  This is a CCITT V.35 (ISO 2593) connector.  Frequently used for X.25 networking.

V.35 Pinout

  Pin      Name      Function
Pin Name Function H DTR Data terminal ready P TXA Transmit data A S TXB Transmit data B U SCTEA Serial clock transmit external A W CTEB Serial clock transmit external B C RTS Request to send E DSR Data set ready R RXA Receive data A T RXB Receive data B V SCRA Serial clock receive A X SCRB Serial clock receive B Y SCTA Serial clock transmit A AA SCTB Serial clock transmit B A Shield Frame ground or shield B SG Signal ground D CTS Clear to send F DCD Data carrier detect J RI Ring indicator


Now, how do you make it play?  I have actually tested most of these configurations.  All you need is a large spool of Cat 5 wire, appropriate crimpers and ends, and endless time.  Enjoy!

Cat 5 Cable

The following colors the most commonly used in Cat 5 cables and will be used in the diagrams on this page. If your cable has different colors, you will need to adapt accordingly.

           White / Blue
           White / Green
           White / Brown
           White / Orange

MMJ Cable

The standard MMJ cable has six wires and is flat, not round.

10Base-T straight through cable

The "straight through" cable is used to connect a network device (LAN card, etc.) to a hub or switch.  This cable has TIA/EIA T568B connectors on each end (hey, they look like RJ-45's to me!).  Each connector is wired like this:

    Pin    Wire
      1    White / orange
      2    Orange
      3    White / green
      4    Blue
      5    White / blue
      6    Green
      7    White / brown
      8    Brown

10Base-T crossover cable

This cable is used to connect two identical devices.  You can connect two LAN cards together to create a miniature network.  Or you can connect two hubs together to expand your network.  Technically, one connector is TIA/EIA T568B, and the other is TIA/EIA T568A.  Take a look:

    Connector 1          Connector 2
    Pin    Wire                    Pin    Wire
      1    White / orange            1    White / green
      2    Orange                    2    Green
      3    White / green             3    White / orange
      4    Blue                      4    Blue
      5    White / blue              5    White / blue
      6    Green                     6    Orange
      7    White / brown             7    White / brown
      8    Brown                     8    Brown

When I make one of these, I use a red Sharpie marker and put a band near each end so I won't confuse it with a straight through cable.  Your mileage may vary.

FDDI crossover cable

Wait, I thought FDDI used fiber!  Well, yes, but there's a version of FDDI that uses UTP – which is good old Cat 5 cable with RJ-45's.  It's also known as CDDI, or "FDDI over copper". The only reason you'd need a crossover cable is because you're connecting two FDDI cards together.  I had to do this, so just in case you need to know:

    Connector 1                   Connector 2
    Pin    Wire             Function        Pin  Wire
      1    White / orange   Tx+               1    White / brown
      2    Orange           Tx-               2    Brown
      3    White / green                      3    White / green
      4    Blue                               4    Blue
      5    White / blue                       5    White / blue
      6    Green                              6    Green
      7    White / brown    Rx+               7    White / orange
      8    Brown            Rx-               8    Orange

The colors used here are not standardized as they are for 10Base-T.  I just used the 10Base-T standard for the first connector.  FDDI requires that the outside two pairs are swapped, but the other pairs are straight through.  The good thing about standards is there are so many to choose from!


It seems every piece of Digital equipment has at least one MMJ connector. Here's how to make a "straight through" MMJ cable.  The colors are my own choice.

    1 Orange               1 White / orange
    2 Blue                 2 Green
    3 White / blue         3 White / green
    4 White / green        4 White / blue
    5 Green                5 Blue
    6 White / orange       6 Orange

There's no "crossover" MMJ cable – all MMJ cables are "straight through".  I said that it was a simpler idea.  

RJ-45 to DB-9 Female

This hooks a PC's serial port to a DEC terminal server.  I left out the "handshaking" lines (RTS, CTS, DSR, DTR, DCD, RI).

    RJ-45                      DB-9 Female
    1  White / blue            1  No connection
    2  Blue                    2  Green
    3  White / green           3  Blue
    4  Brown                   4  No connection
    5  White / brown           5  White / green and White / blue
    6  Green                   6  No connection
    7  White / orange          7  No connection
    8  Orange                  8  No connection
                               9  No connection

Digital made an adapter, H8585-AA, with the following pinouts.  Could this be the full-handshaking version of the above?  I haven't tested it, but it would seem likely.

    RJ-45                      DB-9 Female
    1  White / blue            1  Connect to pin 6 and White / orange
    2  Blue                    2  Green
    3  White / green           3  Blue
    4  Brown                   4  Orange
    5  White / brown           5  White / green and White / blue
    6  Green                   6  Connect to pin 1 and White / orange
    7  White / orange          7  Brown
    8  Orange                  8  White / brown
                               9  No connection

RJ-45 to DB-25 Male

This hooks a DEC terminal server to a modem.  The handshaking lines are used in this one.  Unfortunately, there's no place on a RJ-45 for DCD or RI.  Digital made an adapter for this called the H8585-AC.

    RJ-45                  DB-25 Male
    1  White / blue         2  Green
    2  Blue                 3  Blue
    3  White / green        4  White / brown
    4  Brown                5  Brown
    5  White / brown        6  Orange
    6  Green                7  White / green and White / blue
    7  White / orange      20  White / orange
    8  Orange

Digital's DECserver 900TM could be configured to bring those signals out on the RJ-45 instead of DSR and CTS.  They made an adapter, the H8585-AB, which had these pinouts:

    RJ-45                     DB-25 Male
    1  White / blue            2  Green
    2  Blue                    3  Blue
    3  White / green           7  White / green and White / blue
    4  Brown                   8  Orange
    5  White / brown          20  White / orange
    6  Green                  22  Brown
    7  White / orange         23  White / brown
    8  Orange

RJ-45 to MMJ

This is probably the hardest cable to make.  That's because both of these are crimp-on connectors.  First, the pinouts:

    RJ-45                   MMJ
    1  White / blue         1  Orange and brown
    2  Blue                 2  Blue
    3  White / green        3  White / blue
    4  Brown                4  White / green
    5  White / brown        5  Green
    6  Green                6  White / orange
    7  White / orange
    8  Orange

See how the first wire in the MMJ needs to connect to two wires on the RJ-45? What I do is cut off about 2 inches of the outer cover on the cable near the MMJ end (before I put on the MMJ!). I cut off the White/brown wire (it doesn't go to the MMJ), and strip all the insulation off the Brown wire. I then carefully (very carefully) strip about 1/2 inch of insulation off the Orange wire. If you do this right, the Orange wire's insulation just slips up. I then wrap the two wires together. I put the outer insulation back on, less about 1/2 inch. A piece of electrical tape makes it secure. I then put on the MMJ as indicated.

Digital's BN24H cable was similar to the above, but without the brown wire being hooked to pin 1 of the MMJ.

Digital did make an adapter, the H8584-AC, with the following pinouts. This is a RJ-45 plug and MMJ socket.

       RJ-45                   MMJ Socket
    1  White / blue           1  White / orange
    2  Blue                   2  Green
    3  White / green          3  White / green
    4  Brown                  4  White / blue
    5  White / brown          5  Blue
    6  Green                  6  Orange and brown
    7  White / orange
    8  Orange

RJ-45 (DECserver) to RJ-45 (Cisco router)

To connect a Cisco router's console port to a DECserver terminal server :

    RJ-45 (DECserver)         RJ-45 (Cisco)
    1  White / blue           1  Brown
    2  Blue                   2  Orange
    3  White / green          3  Blue
    4  Brown                  4  White / blue
    5  White / brown          5  White / green
    6  Green                  6  Green
    7  White / orange         7  White / orange
    8  Orange                 8  White / brown

RJ-45 (FlowPoint) to DB-9 Female

Here's how to connect a FlowPoint router's console port to a PC's serial port.

    RJ-45                     DB-9
    1  White / blue           1  no connection
    2  Blue                   2  White / blue
    3  White / green          3  Brown
    4  Brown                  4  no connection
    5  White / brown          5  White / brown
    6  Green                  6  no connection
    7  White / orange         7  Blue
    8  Orange                 8  Green
                              9  no connection

MMJ to DB-25 Female

This hooks an older PC's serial port to a Digital MMJ device (such as a VT220 terminal).  Digital made an adapter called a H8575-A with these pinouts:

  MMJ                      DB-25 Female
  1  Orange                2 Blue
  2  Blue                  3 Green
  3  White /blue           4 Connect to pin 5
  4  White /green          5 Connect to pin 4
  5  Green                 6 Connect to pin 8 and White / orange
  6  White / orange        7 White / blue and White / green
                           8 White / orange and pin 6
                          20 Orange

MMJ to DB-9 Female

This hooks a PC serial port to a Digital MMJ device (such as a VT220 terminal). Digital made an adapter called a H8571-J with these pinouts:

  MMJ                     DB-9 Female
  1 Orange              1 Connect to pins 6 and 8 and White / orange
  2 Blue                2 Green
  3 White / blue        3 Blue
  4 White / green       4 Orange
  5 Green               5 White / blue and White / green
  6 White / orange      6 Connect to pins 1 and 8 and White / orange
                        7 No connection
                        8 Connect to pins 1 and 6 and White / orange
                        9 No connection

MicroVAX System Console Port

The pinouts for early MicroVAX systems do not follow the scheme above. See the VMS FAQ for details.

DB-9 Male (Intermec) to DB-9 Female

This connects a PC serial port to an Intermec 9154 scanner management port (Intermec cable number 060728-00028).

  DB-9 Male                 DB-9 Female
  1  No connection          1  White / blue
  2  Blue                   2  GGreen
  3  Green                  3  Blue
  4  No connection          4  No connection
  5  Orange                 5  White / green
  6  No connection          6  No connection
  7  White / green          7  No connection
  8  No connection          8  Orange
  9  White / blue           9  No connection 

If you have any questions or need to obtain cables or connectors listed on this page, please email Quayle Consulting!

If you need a cable for a device not specified on this page, you must supply the pinouts of the devices!

The The VMS FAQ has lots of good info.

This DEC cable manual is handy (vaxarchive.org).

NullModem.com has lots of pinouts

More DECconnect adapters are here (lammertbies.nl)

Lots of pinouts at AirBorn

For more information about our products and services, including modernization of SPARC, Alpha, VAX, and PDP-11 system hardware, contact us TODAY!

<-- previous code -->