Chapter 7

VT125 Graphics Terminal


The VT125 Graphics Terminal combines a bit map graphics architecture and the versatile alphanumeric features of the VT100 Video Terminal. It is designed for use in business graphics, and laboratory and scientific applications. As a financial analysis or business data tool, the VT125 is capable of plotting trend lines, bar charts and pie charts. In engineering laboratory applications, it can be used for point plot graphs, and strip charts. To support programs written for the VT105, the VT125 includes a VT105 emulator.

The VT125 is a microprocessor-based terminal that directly executes Digital's general-purpose graphics descriptor, ReGIS (Remote Graphics Instruction Set). ReGIS allows the creation and storage of pictorial data as simple ASCII text. ReGIS is designed for efficient communication of graphics data to remote displays.

As a companion product to the VT125 terminal, Digital offers the Letterprinter 100 (Chapter 10), the LA50 Personal Printer (Chapter 12), or the DECwriter Correspondent (Chapter 13). Together, the terminal and printer provide you with a complete graphics package that is both powerful and easy to use.

Currently available models of the VT125 include:

VT100/graphics CRT with EIA interface, 120 Vac, 60 Hz
VT100/graphics CRT with EIA interface, 240 Vac, 50 Hz
VT125-AA with word processing keyboard
VT125-AB with word processing keyboard

Major Features

The following features are unique to the VT125 Graphics Terminal:

Universal VT100 Features
The VT125 terminal incorporates all of the basic VT100 features. These features are described in detail in Chapter 5.
Bit Map Graphics Architecture
The bit map architecture of the VT125 allows you to display many things that are difficult or impossible to display on an ordinary CRT, such as pie charts, bar graphs, trend lines, and special symbols. In graphics mode each pixel on the screen is controlled by bit map memory which is manipulated under program control.
Screen Resolution 768 by 240 Pixels
The VT125 display is made up of 240 rows by 768 dots. These dots (picture elements or pixels) can be controlled individually by your applications' programs.
Two Full Graphics Planes
This feature allows the terminal to display images one at a time; two at a time, for comparison; or together to achieve varying shades of gray or color.
VT105 Emulation Mode
The VT125 Graphics Terminal will support software written for a VT105 Terminal. This saves the time and expense of converting existing VT105 programs to ReGIS.
Auxiliary Port
This feature allows for the connection of any EIA auxiliary device to the VT125. The primary purpose of this port is to connect a Letterprinter 100, LA50 Personal Printer, or DECwriter Correspondent printer for hardcopy of the image on the VT125 screen. The auxiliary port can communicate with the auxiliary device in 7- or 8-bit ASCII and is capable of communicating at one of four baud rates – 300, 1200, 2400, or 9600. The EIA RS-232-C port is bidirectional.
Color Output for RGB Monitor
This feature allows you to get color output when using the VT125. The VT125 has the capability of displaying 64 colors, four at a time on the color monitor.
ReGIS firmware
ReGIS firmware provides the ability for direct execution of ReGIS data plotting commands. The firmware interprets simple ASCII text and converts it into pictures on the video screen using lines, circles, and curves.


Graphics capabilities in the ReGIS firmware include:


ReGIS, a graphic descriptor, is a symbol system that describes the parts of a picture. A picture (graphic) is the best way to communicate some types of information. A requirement for a graphic descriptor is to describe the picture with as few symbols as possible to save storage space and communication line time. ReGIS works by considering a picture to be a group of graphic objects. Each graphic object is a standard geometric shape that can be described with a few characters of information. For example, ReGIS understands that the shape of a circle applies to any circle that can be drawn. A circle can be described on the screen by the location of its center and a point on its circumference. The same type of understanding for other graphic objects allows ReGIS to be a compact descriptor of graphic images.

The VT125 Graphics Option can display 184,320 pixels in a 768-by-240-by-2 array and is considered a medium resolution device. The Graphic Option memory can store 196,608 pixels in a 768-by-256-by-2 array. The display of the VT125 is a window that looks into this memory. The window can move in any direction with a screen command to display all parts of the pixel memory, or move an object in the image to the center of the display.


The VT125 Graphics Terminal is a VT100 text terminal with a factory installed Graphics Option. Other options are listed below.

Part Number Description
VT1XX-AB Advanced video option for VT100 or VT125. Adds the following features to the text terminal capabilities of the VT125: four alternate character attributes – bold, underline, reverse video, and blink – which can be selected in any combination on a character-by-character basis; extra memory to display a total of 24 lines in either 80 or 132 column format; provision for alternate character sets, which may also be selected on a character-by-character basis.
VT1XX-CA VT101/VT102/VT131/VT125 20 mA current loop option with cable
VT1XX-CE Upgrade kit to convert your VT100 or VT125 to word processing keyboard.

The printer port VT1XX-AC cannot be used on the VT125 and is not required since a printer port is included with the VT125.

Accessories and Supplies

The following VT125 accessories and supplies are available. Check with your sales representative of Digital's Installed Base Group for the latest information.


Part Number Description
4A-VT125-00 VT125 spares kit

Controls and Indicators

The VT125 has the same monitor, keyboard, and audible and visual controls and indicators as other members of the VT100 family. See Chapter 5 or VT125 User Guide.

Operator Features

All VT100 family members incorporate features that can be selected in set-up. Set-up features determine how the terminal operates. They allow the terminal to be configured according to your preference, and provide compatibility to the host and AC power source. Unlike some terminals, the VT100 family does not use switches or jumpers to individually turn on or off the built-in features. Instead, these terminals use a nonvolatile memory (NVR) that always remembers what features have been selected, just as if a switch had been set.


Set-Up mode provides two brief descriptions of the current feature status. Set-Up A displays the location of the tab stops set in the terminal and a visual ruler that numbers each character position on the line. The status of other terminal features is summarized in Set-Up B.

See Chapter 5 for details on entering Set-Up A.

As described in Chapter 5, Set-Up B mode only can be entered from Set-Up A Mode. Because the VT125 Graphics Terminal has features not included with a regular VT100 terminal, Set-Up B has additional features for you to select.

In addition to all the set-up features described in Chapter 4 for the VT100 family, the VT125 has two additional set-up features.

Aux Port Bits Per Char

This switch sets the auxiliary port to match the character length of your printer or other serial device. When set to eight bits, the eighth bit is a space. (This is the same as 7S or 8N on other vendors' terminals.) Note: Only on VT125.

Aux Port Speed

This sets the auxiliary port to one of four baud rates: 300, 1200, 2400, and 9600. Set the speed to match the speed of your auxiliary device. Note: Only on VT125.

Figure 7-1 Summary of Set-Up B Mode for VT125


The VT125 is a serial line interface video terminal. It communicates asynchronously over full-duplex lines. All communications features are keyboard-selectable. Transmit and receive speeds can be set independently at 16 different rates from 50 to 19,200 baud. An EIA RS232-C compatible interface is standard. A 20 mA current loop interface is optional.

The VT125 Graphics Terminal can be connected to a computer directly or through a common carrier facility (telephone line). In both applications, either the Electronic Industry Association (EIA) interface provided with the terminal or the 20 mA current loop interface option are used.

When connecting the terminal to the computer through a telephone line, a modem or acoustic coupler is needed. The modem or acoustic coupler changes the serial characters transmitted between terminal and computer into signals that can be transmitted over the telephone line. Several types of modems can be used with the VT125 terminal. However, the modem used by the terminal must be compatible with the modem used by the computer.

Programming Information

The VT125 Terminal responds to three different programming standards:

Consult Appendix D or your VT125 User Manual for detailed information on escape sequences and sequence formats.


The VT125 has an automatic internal self-test that checks the terminal for correct operation every time the power is turned on. The VT125 is easy to maintain. No preventative maintenance is necessary. If a repair should be required, Field Service is available worldwide. The terminal is designed to make machine access and the replacement of parts fast and easy.


The VT125 terminal has several self-tests available for checking the operation of the terminal.

To prepare for all the tests except the power-up test, prepare the terminal by first pressing the power switch to the OFF position. Then disconnect the communications cables from the computer and auxiliary data ports. Third, if a cable was connected to the EIA computer port connector, install an EIA loopback connector on the EIA connector. If the cable was connected to the 20 mA connector, then install the loopback connector that was included with that option. Do not use the EIA loopback connector at the same time as the current loopback connector. Finally, turn the power switch ON. Note: To perform auxiliary port tests, consult your VT125 User Guide for details.

Operator Troubleshooting Checklist

If it appears that there is a problem in the terminal, initiate the power-up self-test program. This test will help to determine if the problem lies in your terminal or in some other part of the computer system.

Terminal does not respond to typed characters. Keyclicks are generated and keyboard indicators function.

Additional Documentation

The following documents contain more detailed information about the VT125 family of terminals.

VT125 User Guide (EK-VT125-UG)
Describes the installation, operation, and programming of the VT125 terminal.
VT100 Mini Maintenance Manual (EK-VT100-J1)
Describes procedures to troubleshoot and repair the VT100 and VT125 terminals.
VT100 Technical Manual (EK-VT100-TM)
Describes VT100 and VT125 terminals to detailed block level.
VT125 Illustrated Parts Breakdown (EK-VT125-IP)
Provides a detailed parts breakdown of the VT125 field replaceable units. Does not contain part numbers for components on the printed circuit boards.
VT125 Print Set (MP-01053)
Provides a complete set of electrical and mechanical schematic diagrams for the VT125 terminal.
VT125 Terminal Programming Reference Card (EK-VT125-RC)
Provides a summary of the VT125 escape and control sequences on a pocket-sized reference card.

Also of interest are:

PDP-11 and VAX Systems & Options Catalogs
Provide you with the most accurate and up-to-date information on currently available PDP-11 and VAX systems, options, and software products. These customer documents are designed to help you select the right Digital product for your needs. European versions are also available.

If you require information not contained in these documents, contact your local Digital representative, dealer, or distributor.


For specifications on characteristics common to all VT100 family members, see Chapter 5.

Performance Characteristics

VT125 Graphics format 768 horizontal × 240 vertical addressable points, two planes
768 × 480 logical address range for 2:1 picture aspect ratio

VT125 Graphics Communication

Composite video output

Composite video output provides RS170 output with the following nominal characteristics:

Output impedance Equals 75 ohms. DC coupled sync level = 0 V
Black level Equals approximately 0.3 V when loaded with 75 ohms
White level Equals approximately 1.0 V with a 75 ohm load
Composite sync waveform Meets EIA RS170 standards
Vertical interval composed of six equalizing pulses, six vertical sync pulses, and six more equalizing pulses
Vertical interval timing Equalizing pulse width = 2.33µs ± 50ns
Vertical pulse width = 27.28µs ± 200ns
Horizontal pulse width = 4.71µs ± 50ns
Horizontal blank width = 11.84µs ± 50ns/80 column mode
12.34µs ± 50ns/132 column mode
Front porch = 1.54µs ± 50ns

Video Input (J8)

An analog signal applied to the video input will be "ORed" with the internal video signal so the beam intensity at any point on the screen will correspond to the intensity of that signal. This would tend to make the beam brighter at that point. A video signal on this input affects only the internal screen and does not appear on the composite video output. This input has the following nominal characteristics:

Input impedance 75 ohms, DC coupled
Black level 0 V
White level 1.0 V
Maximum continuous input ± 2.0 V

The external video source must be synchronized to the VT125. It may do this by referencing the composite sync on the composite video output. This means that the video input will not synchronize with any composite video source.

Color Video Output

RGB DC coupled analog signal
Output impedance 75 ohms
Level ranging 0-1 volt
Synchronization None embedded