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# MMJ: DEC's Modified Modular Jack

The VT340 has three MMJ ports: the first and second communication
ports and the printer port. Officially it is called "DEC-423" or
"DECconnect", but colloquially and most commonly it is "MMJ".
The VT340 has three [MMJ][MMJ] ports: the first and second
communication ports and the printer port. They are backwards
compatible with RS-232 but do not use the same connector. Officially
Digital called it **DEC-423** or **DECconnect**, but colloquially and
most commonly it is simply **MMJ**.

<!-- XXX TODO: Put image of back panel here with arrow to MMJ and -->
<!-- showing synonyms: MMJ, DEC-423, DECconnect. -->

Note that using MMJ is not strictly required as the VT340 also has a
standard RS-232 DB-25 connector which can be configured in the VT340
Setup menu to map to the first communication port. However, to get the
most of your VT340, including dual sessions, serial input of
keystrokes, and printing graphics, you'll want MMJ cables.
most of your VT340 — dual sessions, serial input of
keystrokes, and printing graphics — you'll want MMJ cables.

## Why MMJ is nifty

Expand All @@ -22,45 +24,108 @@ keystrokes, and printing graphics, you'll want MMJ cables.
* Based on EIA-423-D, so cables can be much longer than RS-232
* Backwards compatible with RS-232 signals

## MMJ to DE-9F Adapter
## MMJ to RS-232 Adapter

Since modern computers don't use MMJ, you'll need a female DE-9 RS-232
to MMJ socket adapter. According to [Lammert Bies][lammert] the DEC
model number for this adapter is **H8571-J**.
<!-- XXX TODO: Insert picture of adapter here. -->

Connecting to a modern PC requires an adapter. While one can add
RS-232 adapters to the the MMJ ports on the back of the VT340,
hackerb9 has found it easier and more elegant to put an MMJ adapter on
the RS-232 port of the PC. Of course, this requires using MMJ cable
(see "BC16E" below).

The ideal solution is a female DE-9 RS-232 to MMJ port adapter.
[Lammert Bies][lammert] says the DEC model number for the official
adapter is **H8571-J**.

[lammert]: https://www.lammertbies.nl/comm/cable/dec-mmj

<details><summary>

#### DEC H8571-J adapter: PC RS232 serial port to MMJ
### Details on the DEC H8571-J adapter

</summary>
<ul>

DEC's official MMJ to DE-9 adapter for connecting a VT340 to a PC.
Pinout for DEC's official MMJ to DE-9 adapter for connecting a VT340
to a PC, printer, or other RS-232 device.

| MMJ<br/>RS-232 name | MMJ<br/>Pin | DE-9<br/>pin | DE-9<br/>RS-232 name |
|--------------------:|:-----------:|:------------------:|-----------------------------------------------------|
| DTR | 1 | 4 | Data Terminal Ready |
| Tx+ | 2 | 3 | Transmit Data |
| Tx-</br>Rx- | 3<br/>4 | 5 | Ground |
| Rx+ | 5 | 3 | Receive Data |
| DSR | 6 | 1<br/>6<br/>8<br/> | Data Set Ready<br/>Clear To Send<br/>Carrier Detect |
| MMJ name | MMJ Pin | DE-9 pin | DE-9 name |
|--------------------------:|--------:|:------------------:|-----------------------------------------------------|
| [DTR][DTR] | 1 | 4 | Data Terminal Ready |
| [Tx+][Tx+] | 2 | 3 | Transmit Data |
| [Tx-][Tx-]</br>[Rx-][Rx-] | 3<br/>4 | 5 | Ground |
| [Rx+][Rx+] | 5 | 3 | Receive Data |
| [DSR][DSR] | 6 | 1<br/>6<br/>8<br/> | Data Set Ready<br/>Clear To Send<br/>Carrier Detect |


The H8571-J is nearly identical to the hackerb9's wiring, below, with
one minor difference: instead of using DTR (DB9 pin 4) on the PC,
hackerb9's connector uses RTS (DB9 pin 7). Why? Because all modern
UNIX systems can handle RTS/CTS flow control but DTR/DSR support is
iffy. In particular, the Linux kernel still has no support as of 2025.
iffy. In particular, the Linux kernel still lacks support as of 2025.

----------------------------------------------------------------------

</uL></details>

### Hackerb9's Suggestion for DE-9 to MMJ Wiring

Here's how hackerb9 wired up a 9-pin female to MMJ connector so that,
like original DEC equipment, a PC can be plugged into any MMJ device
using just a single cable. In RS232-speak, all devices (VT340, PC,
printer, etc) are "DTE" and all cables are "crossover" (AKA "null modems").

| MMJ function (after cable) | MMJ Socket | Usual color | DE-9 Female | DE-9 RS-232 name |
|---------------------------:|-----------:|---------------|:------------|:---------------------------------|
| Data Terminal Ready | 1 | White | 7 | Request To Send |
| Transmit Data | 2 | Black | 3 | Transmit Data |
| Ground | 3<br/>4 | Red<br/>Green | 5 | Ground |
| Receive Data | 5 | Yellow | 2 | Receive Data |
| Data Set Ready | 6 | Blue | 1<br/>8 | Carrier Detect<br/>Clear To Send |

<ul><i><sub>
Note: Unassembled adapter kits come with only six DSub female pins.</sub><sub>
Take the spare from joining MMJ 3 and 4 and use it so that MMJ 6 can
go to both DE-9 pins 1 and 8.</sub>
<details><summary>Sub-note</summary><sub>
If there was a seventh DSub female pin it could be used for DE-9 pin 6
(Data Set Ready), connected to MMJ pin 6 (Data Set Ready). </sub><sub>
It has been omitted here in favor of DE-9 pins 1 and 8 (Carrier Detect and
Clear to Send).</sub>
<details><summary>Sub-sub-note</summary>

<sub>Perhaps the most important of those is pin 1 (Carrier Detect) as
without it programs like `less` and `mesg` would hang forever on open
of /dev/tty.</sub> <sub>(A software fix if your cable lacks Carrier Detect is to
run `stty clocal`.)</sub> <sub> Pin 8 (Clear To Send) is also useful as it is
common for modern systems to presume hardware flow control (even
though the VT340 does not have it).</sub> <sub>Pin 6 (Data Set Ready) is least
important as [UNIX systems have ignored it for eons][UWR870] in favor
of Carrier Detect (Pin 1). For more considerations, see the [Linux
Text Terminal Howto][TLDPTTH].</sub>

</details>
</details>
</i></ul>

[UWR870]: https://www.washington.edu/R870/TerminalsModems.html
[TLDPTTH]: https://tldp.org/HOWTO/Text-Terminal-HOWTO-12.html


<!-- Note that when assembling, if you follow hackerb9's schematic, you'll
need to cut and splice one of the female D-Sub pins. See the [assembly
instructions](mmj-db9f-assembly.md) for details. -->

Despite the VT340 lacking hardware flow control, this wiring works
well for communication. These words are being typed on a VT340,
flowing over a standard "DEC-423 BC16E" cable, through this homemade
MMJ to DE-9 adapter, and arriving on a UNIX host's serial port.
_Caveat: Some USB to RS232 serial adapters lack "on-chip XON/XOFF" and
will cause dropped characters ("⸮"). See
[flowcontrol.md](flowcontrol.md) for details._

<!-- XXX TODO: Insert picture of adapter here. -->

### MMJ Pin 1 is furthest from "thumb" of MMJ latch
## MMJ Pin outs

In the VT340, DEC assigns the 2nd and 3rd serial-port pins like this
(a 6-pin MMJ DEC-423 port):
Expand All @@ -74,6 +139,8 @@ In the VT340, DEC assigns the 2nd and 3rd serial-port pins like this
6 - DSR
```

> TIP: MMJ Pin 1 is furthest from "thumb" of the MMJ latch
<details><summary><h3>On the naming and numbering of MMJ pins</h3></summary>
<ul>

Expand All @@ -88,7 +155,7 @@ In the VT340, DEC assigns the 2nd and 3rd serial-port pins like this

> Only valid when looking at a port ("jack"), not the plug on a cable.
#### Flip it and reverse it
### Flip it and reverse it

Because every DEC-423 cable is a crossover cable, the functions
associated with the pins swap position, as do the wire colors.
Expand All @@ -105,65 +172,16 @@ associated with the pins swap position, as do the wire colors.

</ul></details>

## Hackerb9's Suggestion for DE-9 to MMJ Wiring

Here's how hackerb9 wired up a 9-pin female to MMJ connector so that,
like original DEC equipment, a PC can be plugged into any MMJ device
using just a single cable. In RS232-speak, all devices (VT340, PC,
printer, etc) are "DTE" and all cables are "crossover" (AKA "null modems").

| MMJ function (after cable) | MMJ Socket | Usual color | DE-9 Female | DE-9 RS-232 name |
|---------------------------:|-----------:|---------------|:------------|:---------------------------------|
| Data Terminal Ready | 1 | White | 7 | Request To Send |
| Transmit Data | 2 | Black | 3 | Transmit Data |
| Ground | 3<br/>4 | Red<br/>Green | 5 | Ground |
| Receive Data | 5 | Yellow | 2 | Receive Data |
| Data Set Ready | 6 | Blue | 1<br/>8 | Carrier Detect<br/>Clear To Send |

<ul><i><sub>
Note: Unassembled adapter kits come with only six DSub female pins.</sub><sub>
Take the spare from joining MMJ 3 and 4 and use it so that MMJ 6 can
go to both DE-9 pins 1 and 8.</sub>
<details><summary>Sub-note</summary><sub>
If there was a seventh DSub female pin it could be used for DE-9 pin 6
(Data Set Ready), connected to MMJ pin 6 (Data Set Ready). </sub><sub>
It has been omitted here in favor of DE-9 pins 1 and 8 (Carrier Detect and
Clear to Send).</sub>
<details><summary>Sub-sub-note</summary>

<sub>Perhaps the most important of those is pin 1 (Carrier Detect) as
without it programs like `less` and `mesg` would hang forever on open
of /dev/tty.</sub> <sub>(A software fix if your cable lacks Carrier Detect is to
run `stty clocal`.)</sub> <sub> Pin 8 (Clear To Send) is also useful as it is
common for modern systems to presume hardware flow control (even
though the VT340 does not have it).</sub> <sub>Pin 6 (Data Set Ready) is least
important as [UNIX systems have ignored it for eons][UWR870] in favor
of Carrier Detect (Pin 1). For more considerations, see the [Linux
Text Terminal Howto][TLDPTTH].</sub>

</details>
</details>
</i></ul>

[UWR870]: https://www.washington.edu/R870/TerminalsModems.html
[TLDPTTH]: https://tldp.org/HOWTO/Text-Terminal-HOWTO-12.html

Despite the VT340 lacking hardware flow control, this wiring works
well for communication. The words you are reading are flowing from a
VT340, over a standard "DEC-423 BC16E" cable, through this homemade
MMJ to DE-9 adapter, to a UNIX host's serial port. _Caveat: Some USB
to RS232 serial adapters lack "on-chip XON/XOFF" and will cause
dropped characters ("⸮"). See [flowcontrol.md](flowcontrol.md) for details._


### Purchasing Unassembled MMJ-DB9F Adapters
## Purchasing Unassembled MMJ-DB9F Adapters

Most of the MMJ-DB9F adapters for sale online come with the pins
disconnected so you can choose how you wish to wire it.

Hackerb9 ordered from Pacific Cable (part no. AD-9FT6-G1D), but [their
website](https://pacificcable.com) is down. It looks like you can get
the same thing from other suppliers, but no promises.
the same thing from other suppliers, but no promises. eBay can be
cheaper but the quality is certainly lower.

* [L-Com Item # REC096FD][lcom].<br/>
_($14 each + $10 shipping in the US as of 2024)._
Expand Down Expand Up @@ -192,24 +210,7 @@ the same thing from other suppliers, but no promises.
[connectzone]: https://www.connectzone.com/ma-09fd.html


<!-- Note that when assembling, if you follow hackerb9's schematic, you'll
need to cut and splice one of the female D-Sub pins. See the [assembly
instructions](mmj-db9f-assembly.md) for details. -->


## More pinouts

### DE-9 Pinout (Female)

[Note: a "DE-9" port is what we used to call a "DB-9" port. Wikipedia
says we were all wrong.]

___________
\5 4 3 2 1/ DE-9
\9 8 7 6/ Female
------- Connector

### BC16E DEC-423 Cable
## BC16E DEC-423 Cable

This is DEC's official MMJ cable.

Expand All @@ -231,7 +232,18 @@ out of ordinary 6-wire telephone cable.

<!-- XXX TODO: Insert picture of crimper and MMJ plugs. -->

----------------------------------------------------------------------
## More pinouts

### DE-9 Pinout (Female)

[Note: a "DE-9" port is what we used to call a "DB-9" port. Wikipedia
says we were all wrong.]

___________
\5 4 3 2 1/ DE-9
\9 8 7 6/ Female
------- Connector


<details><summary>

Expand All @@ -243,7 +255,7 @@ The Linux Documentation Project has a pinout for a cable functionally
similar to the one hackerb9 suggests above. It additionally loops back
the Request to Send (RTS) signal from the PC back into the Carrier
Detect (CD) and Data Terminal Ready (DTR) pins. This seems like a
mistake as RTS and DTR are both _outputs_ pins and one could fry the
mistake as RTS and DTR are both _output_ pins and one could fry the
serial port if they disagree about what voltage to set the line.

His schematic is:
Expand All @@ -268,8 +280,6 @@ hardware damage.
</details>




<details><summary>

## Other DEC pinouts from the Digital Unix FAQ
Expand All @@ -295,16 +305,17 @@ Digital carries four DB-to-MMJ adaptors. They are internally wired as follows

</blockquote>

_[Note from hackerb9: Clearly the DEC FAQ needs to be updated as it is
missing H8571-J and perhaps others.]_

----------------------------------------------------------------------

</details>


<details><summary>

## Pinout of 9-pin and 25-pin serial connectors

</summary>
<details>
<summary><h3>Pinout of 9-pin and 25-pin serial connectors</h3></summary>

Adapted from the Linux Serial HOWTO chapter 19.

Expand Down Expand Up @@ -361,4 +372,4 @@ renaming it "Ready To Receive".
[Rx+]: ## "Receive Data positive"
[Rx-]: ## "Receive Data negative"
[RI]: ## "Ring Indicator"

[MMJ]: ## "Modified Modular Jack"

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