The VT340 has three 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.
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 — dual sessions, serial input of keystrokes, and printing graphics — you'll want MMJ cables.
- Simple "telephone" line cable
- Connect any two devices with that one cable (no more messing with null modems to convert DTE/DCE)
- Quick connect/disconnect
- Based on EIA-423-D, so cables can be much longer than RS-232
- Backwards compatible with RS-232 signals
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 would have a female DE-9 RS-232 port to plug into modern PCs. Lammert Bies says the DEC model number for that adapter is H8571-J. However, if you are going to make your own, consider hackerb9's variation, below.
Details on the DEC H8571-J adapter
Pinout for DEC's official MMJ to DE-9 adapter for connecting a VT340 to a PC, printer, or other RS-232 device.
| MMJ name | MMJ Pin | DE-9 pin | DE-9 name |
|---|---|---|---|
| DTR | 1 | 4 | Data Terminal Ready |
| Tx+ | 2 | 3 | Transmit Data |
| Tx- Rx- |
3 4 |
5 | Ground |
| Rx+ | 5 | 3 | Receive Data |
| DSR | 6 | 1 6 8 |
Data Set Ready Clear To Send 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 lacks support as of 2025.
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 4 |
Red Green |
5 | Ground |
| Receive Data | 5 | Yellow | 2 | Receive Data |
| Data Set Ready | 6 | Blue | 1 8 |
Carrier Detect Clear To Send |
-
Note: Unassembled adapter kits come with only six DSub female pins.
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-note
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). It has been omitted here in favor of DE-9 pins 1 and 8 (Carrier Detect and Clear to Send).Sub-sub-note
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. (A software fix if your cable lacks Carrier Detect is to
run stty clocal.) 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). Pin 6 (Data Set Ready) is least
important as UNIX systems have ignored it for eons in favor
of Carrier Detect (Pin 1). For more considerations, see the Linux
Text Terminal Howto.
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 for details.
In the VT340, DEC assigns the 2nd and 3rd serial-port pins like this (a 6-pin MMJ DEC-423 port):
_______________ 1 - DTR
| | 2 - TxD+
| 1 2 3 4 5 6 | 3 - TxD-
|________ _| 4 - RxD-
|___| 5 - RxD+
6 - DSR
TIP: MMJ Pin 1 is furthest from the "thumb" of the MMJ latch
On the naming and numbering of MMJ pins
| Pin | DEC-423 name | RS-232 name | Wire Color | DE-9 | DB-25 |
|---|---|---|---|---|---|
| 1 | Ready Out | DTR | White | 4 | 20 |
| 2 | Receive Data + | TxD | Black | 3 | 2 |
| 3 | Receive Data - | GND | Red | 5 | 7 |
| 4 | Transmit Data - | GND | Green | 5 | 7 |
| 5 | Transmit Data + | RxD | Yellow | 2 | 3 |
| 6 | Ready In | DSR | Blue | 6 | 6 |
Only valid when looking at a port ("jack"), not the plug on a cable.
Because every DEC-423 cable is a crossover cable, the functions associated with the pins swap position, as do the wire colors.
| MMJ pin | Function at MMJ port | Function at cable plug |
|---|---|---|
| 1 | DSR | DTR |
| 2 | RxD | TxD |
| 3 | GND | GND |
| 4 | GND | GND |
| 5 | TxD | RxD |
| 6 | DTR | DSR |
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 is down. It looks like you can get the same thing from other suppliers, but no promises. eBay can be cheaper but the quality is certainly lower.
-
L-Com Item # REC096FD.
($14 each + $10 shipping in the US as of 2024).Surprisingly, L-Com also offers a free 3D CAD model and blueprint if you want to make your own.
-
Stonewall Cable SKU: P-MMJ-DB9F.
($15 each + $20 shipping in the US as of 2024). -
Connect Zone SKU MA-09FD.
($5 each + $10 shipping in the US as of 2024)Connect Zone may no longer be in business as their website is suffering badly from bitrot.
This is DEC's official MMJ cable.
| MMJ Plug 1 | Wire Color | DEC-423 | DEC-423 | Wire Color | MMJ Plug 2 |
|---|---|---|---|---|---|
| 1 | White | Rdy Out | Rdy In | Blue | 1 |
| 2 | Black | Tx+ | Rx+ | Yellow | 2 |
| 3 | Red | Tx- | Rx- | Green | 3 |
| 4 | Green | Rx- | Tx- | Red | 4 |
| 5 | Yellow | Rx+ | Tx+ | Black | 5 |
| 6 | Blue | Rdy In | Rdy Out | White | 6 |
Instead of using vintage cables which cost a premium, hackerb9 purchased an MMJ crimper and a bag of MMJ plugs to make BC16E cables out of ordinary 6-wire telephone cable.
[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
Mark Gleaves MMJ to 9-pin serial with fake RTS/CTS flow control
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 output pins and one could fry the serial port if they disagree about what voltage to set the line.
His schematic is:
DEC MMJ Linux PC DB9
Pin Signal Signal Pin
=== ====== ====== ===
1 DTR -----------------------|---> DSR 6
|---> CTS 8
2 TxD ---------------------------> RxD 2
3 SG (TxD)--------------------|--- SG 5
4 SG (RxD)--------------------|
5 RxD <--------------------------- TxD 3
6 DSR <-----------------------|--- RTS 7
|--> DTR !? 4
|--> CD 1
(no connection) RI 9
Hackerb9 does NOT RECOMMENDED this cable due to the possibility of hardware damage.
Other DEC pinouts from the Digital Unix FAQ
This describes the 6-pin modified modular jack (MMJ) used for serial ports on various Digital hardware.
Digital carries four DB-to-MMJ adaptors. They are internally wired as follows
Adaptor Gender 1 2 3 4 5 6 Use with: RdyOut TX+ TX- RX- RX+ RdyIn H8575-A F 20 2 7 7 3 6&8 VTxxx terminal H8571-C M 6 3 7 7 2 20 Digital printer H8571-D M 6 3 7 7 2 20 Modem H8571-E M 20 2 7 7 3 6&8 LaserWriter
[Note from hackerb9: Clearly the DEC FAQ needs to be updated as it is missing H8571-J and perhaps others.]
Pinout of 9-pin and 25-pin serial connectors
Adapted from the Linux Serial HOWTO chapter 19.
The pin numbers are often engraved in the plastic of the connector but you may need a magnifying glass to read them. Note DCD is sometimes labeled CD. The numbering of the pins on a female connector is read from right to left, starting with 1 in the upper right corner (instead of 1 in the upper left corner for the male connector as shown below). --> direction is out of PC.
___________ ________________________________________ \1 2 3 4 5/ Looking at pins \1 2 3 4 5 6 7 8 9 10 11 12 13/ \6 7 8 9/ on male connector \14 15 16 17 18 19 20 21 22 23 24 25/ ------ -----------------------------------
DB-9 DB-25 Name Full-Name Dir What-it-May-Do/Mean 1 8 DCD Data Carrier Detect <-- Modem online 2 3 RxD Receive Data <-- Receives bytes on PC 3 2 TxD Transmit Data --> Transmits bytes from PC 4 20 DTR Data Terminal Ready --> PC says, "I'm here." 5 7 SG Signal Ground --- 6 6 DSR Data Set Ready <-- Other side is connected 7 4 RTS Request To Send --> "I'm ready to receive" 8 5 CTS Clear To Send <-- PC is clear to send 9 22 RI Ring Indicator <-- Telephone line ringing Note that in modern usage, "Request to Send" is a misnomer from the days of half-duplex. Since the 1980s, the RTS pin has been used for full-duplex "RTS/CTS hardware handshaking". Some have suggested renaming it "Ready To Receive".