Digital VT320 Serial Connections

The VT320 terminal has three serial connections on the back. There are two 6-pin MMJ connections, and a 25-way D-type connector. Some U.S. models of the VT320 don't have the 25-way D-type. This document describes these connectors.

Modified Modular Jack (MMJ) Connections

The VT320 has two MMJ connectors, one for host connections, marked with two arrows, and one for printers with serial interfaces. These connections are physically and electrically identical.

Physical Interface

The leftmost picture shows the host connector on the back of the VT320. The printer connector is identical (though the icon above is different). The picture on the right shows a Digital H8572 adapter end-on. This adapter has two MMJs, for joining lengths of terminal cable. Despite the difference in appearance, they both receive the same shape plugs.

The MMJ and its complementary connector, the Modified Modular Plug (MMP), have an offset tab at the bottom to make sure that they can't be confused with U.S. telephone connections, which have a centred tab. Telephone connections in the U.K. use sockets which are different from both the U.S. design and the MMJ.

The pins in these sockets are numbered from pin 1 on the left to pin 6 on the right.

Electrical Interface

Digital designates the electrical interface as DEC-423. This is a superset of the EIA RS-423-A standard[1], with better filtering and tighter level tolerances. Digital claim reliable data communication with unshielded cables over much greater distances than EIA-232 allows, typically 300m. DEC-423 is compatible with EIA-232 interfaces for connections employing data lines only.

The signals on these pins are described in the table below, lifted directly from EK-VT320-UU-001, Installing and Using the VT320 Video Terminal.

6-Pin DEC-423 Comm and Printer Interface Signals
Pin Signal Mnemonic Description
1 Data terminal ready DTR From VT320
When on, tells the modem or printer that the VT320 is ready to send or receive.
2 Transmitted data TXD+ From VT320
Sends serial characters. Held in the mark state (-) when characters are not being sent.

In modem control modes, sends data only when DSR and DTR signals are on.

3 Transmit signal ground TDX- Provides the common ground reference potential for transmitted signals TXD+ and DTR.
4 Receive signal ground RXD- Provides the common ground reference potential for received signals RXD+ and DSR.
5 Received data RXD+ To VT320
Receives serial characters.
6 Data set ready DSR To VT320
For the comm line: When on, tells the VT320 that the modem is in the data mode and is ready to communicate.

For the printer line: Receives DTR on this line. If DSR is present at power-up, the printer controls print operations. If DSR is not present at power-up, the terminal checks for DSR before each print operation.

25-way D-sub Connection

The VT320 provides a second host connector. The Communications Set-Up page has a setting called Host Port Selection which allows use of either the MMJ or this port. The VT320 is a single-session terminal, so only one can be used at a time, though the MMJ printer connector can be used with either type of host connector selected.

Physical Interface

The second host connector is provided on VT320s is a male 25-way D-type subminiature connector.

Electrical Interface

This interface operates in accordance with EIA RS232-C. The table below is also taken directly from Installing and Using the VT320 Video Terminal.

25-Pin RS232-C Comm Port Interface Signals
Pin Signal Mnemonic EIA/CCITT/DIN Description
2 Transmitted data TXD BA/103/D1 From VT320
Sends serial characters. Held in mark state when characters are not being sent.

In modem control modes, sends data only when RTS, CTS, DSR, and DTR signals are on.

3 Received data RXD BB/104/D2 To VT320
Receives serial characters. In modem control modes, ignores characters if RLSD signal is off.
4 Request to send RTS CA/105/S2 From VT320
When on, places the modem in transmit mode.
5 Clear to send CTS CB/106/M2 To VT320
When on, tells the VT320 that the modem is ready to send.
6 Data set ready DSR CC/107/M1 To VT320
When on, tells the VT320 that the modem is in data mode and is ready to exchange RTS, CTS and RLSD signals.
7 Signal ground SGND AB/102/E2 Serves as common ground reference potential for all connector signals, except protective ground.
8 Receive line signal detect (carrier detect) RLSD CF/109/M5 To VT320
When on, tells the VT320 that the signal received on the communication line is good enough to ensure correct demodulation of received data.

When off, indicates no signal received, or signal is unsuitable for demodulation.

12 Speed indicator SPDI CI/112/M4 To VT320
When on, enables a modem to control the terminal's transmit and receive speeds. Sets the speed to 1200 bits per second, regardless of set-up selection.
20 Data terminal ready DTR CD/108.2/S1.2 From VT320
When on, tells the modem that the terminal is ready to send or receive.
23 Speed select SPDS CH/111/S4 From VT320
When on, tells the modem that the receive speed selected in set-up is greater than 600 bits per second.

Other Resources


1 The Telecommunications Industry Association (TIA) web site is the place to look for the latest versions of EIA/TIA standards. At the time of writing (October 1998), the latest versions of EIA-232 and EIA-423 were:

EIA is the Electronic Industries Alliance.