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".
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.
- 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
Since modern computers don't use MMJ, you'll need a female DE-9 RS-232 to MMJ socket adapter. According to Lammert Bies the DEC model number for this adapter is H8571-J.
However, that adapter's wiring (see below) requires an additional null modem to work properly, losing one of the advantages of using MMJ.
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. Since both the VT340 and a typical PC serial port are "DTE", some wires crossover as with a null modem.
| MMJ RS-232 name | MMJ Pin | DE-9 pin | DE-9 RS-232 name | ||
|---|---|---|---|---|---|
| Data Terminal Ready | 1 | → | 1 6 8 |
Carrier Detect Data Set Ready Clear To Send |
White |
| Transmit Data | 2 | → | 2 | Receive Data | Black |
| Ground | 3 4 |
— | 5 | Ground | Red Green |
| Receive Data | 5 | ← | 3 | Transmit Data | Yellow |
| Data Set Ready | 6 | ← | 7 | Request To Send | Blue |
Note: On the DE-9 end of the adapter there is a small problem since
unassembled kits come with only six DSub female pins. That means only
two of pins 1, 6, and 8 can be connected. 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
you have such a cable, 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. 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 for details.
Most of the MMJ-DB9F adapters 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.
-
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.
[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
In the VT340, DEC assigns the 2nd and 3rd serial-port pins like this (a 6-pin MMJ DEC-423 port, similar to EIA-423-D):
_______________ 1 - DTR
| | 2 - TXD+
| 1 2 3 4 5 6 | 3 - TXD-
|________ _| 4 - RXD-
|___| 5 - RXD+
6 - DSR
| MMJ Pin | DEC-423 name | Signal | Wire Color at Jack | DB-25 |
|---|---|---|---|---|
| 1 | Rdy Out | DTR | White | 20 |
| 2 | Tx+ | TxD | Black | 2 |
| 3 | Tx- | GND | Red | 7 |
| 4 | Rx- | GND | Green | 7 |
| 5 | Rx+ | RxD | Yellow | 3 |
| 6 | Rdy In | DSR & DCD | Blue | 6 & 8 |
Note: wire color gets flipped on one plug since DEC-423 always uses crossover cables.
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 | 6 |
| 2 | Black | Tx+ | Rx+ | Yellow | 5 |
| 3 | Red | Tx- | Rx- | Green | 4 |
| 4 | Green | Rx- | Tx- | Red | 3 |
| 5 | Yellow | Rx+ | Tx+ | Black | 2 |
| 6 | Blue | Rdy In | Rdy Out | White | 1 |
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.
The Linux Documentation Project has a pinout very 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 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.
DEC's official MMJ to DE-9 adapter. While physically this looks the same as hackerb9's suggested dongle above, the wiring does not include the necessary "null modem" crossover for connecting a VT340 to a PC.
| MMJ RS-232 name |
MMJ Pin |
DE-9 pin |
DE-9 RS-232 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 |
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
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".