VT5x

Introduction

The VT50 series was a successor to the first DEC-made glass TTY, the VT05. The VT50 was announced in 1974 and provided 12 lines of 80 column text. It handled a limited set of control sequences to position the cursor etc. The VT50 was soon followed by the VT52 which handled 24 lines of text by doubling the built in memory. The VT55 provided simple graphics drawing by adding another board inside the terminal. The VT61 and VT62 also had the same exterior but were block mode terminals. The VT52 was DEC's first upper/lower case video terminal. The VT05 and VT50 terminals were upper-case only.

There were an option of having an internal photo-copier to make a screen copy on paper. The VT50 only had current loop interface while VT52 and onwards had a small replaceable module that could either be EIA RS-232 or current loop. The VT50 was a CRT-based computer terminal introduced by Digital Equipment Corporation (DEC) in July 1974. It provided a display with 12 rows and 80 columns of upper-case text, and used an expanded set of control characters and forward-only scrolling based on the earlier VT05. DEC documentation of the era refers to the terminals as the DECscope, a name that was otherwise almost never seen. The VT50 was sold only for a short period before it was replaced by the VT52 in September 1975. The VT52 provided a screen of 24 rows and 80 columns of text and supported all 95 ASCII characters as well as 32 graphics characters, bi-directional scrolling, and an expanded control character system. DEC produced a series of upgraded VT52's with additional hardware for various uses. The VT52 family was followed by the much more sophisticated VT100 in 1978.

The VT50 supported asynchronous communication at baud rates up to 9600 bits per second and did not require any fill characters. Like other early DEC terminals, the VT50 series were equipped with both an RS-232 port as well as a 20mA current loop, an older serial standard used with teletype machines that was more suitable for transmission over long runs of twisted-pair wiring. Data was read into a small buffer, which the display hardware periodically read to produce the display. Characters typed on the keyboard were likewise stored in a buffer and sent over the serial line as quickly as possible. To interpret the commands being sent in the serial data, it used a primitive central processing unit (CPU) built from small-scale-integration integrated circuits. It examined the data while the display hardware was inactive between raster scan lines, and then triggered the display hardware to take over at the appropriate time. The display system returned control to the CPU when it completed drawing the line.

The CPU was so basic that addition and subtraction could only be done by repeatedly incrementing or decrementing two registers. Moreover, the time taken by such a loop had to be nearly constant, or text lower on the screen would be displayed in the wrong place during that refresh.One notable feature of the VT50 was the introduction of a separate function keypad with the "Gold Key", which was used for editing programs like WPS-8, KED, and EDT. Pressing the Gold Key and then typing one of the keys on the keyboard sent a command sequence back to the host computer.

DEC also offered an optional hard-copy device called an electrolytic copier, which fit into the blank panel on the right side of the display. This device was able to print, scan-line by scan-line, an exact replica of the screen onto a damp roll of special paper. It did this by electroplating metal from an electrode into the paper. The paper ran between two electrodes. The electrode on one side was a thin straight bar oriented across the paper width. The electrode on the other side was a thin helical bar wrapped around a rotating drum. One rotation of the drum scanned an intersecting area of the electrodes across the width of the paper. While the copier did an admirable job of capturing the contents of the screen, the output of the copier had an unfortunate resemblance to wet toilet tissue. Digital patented the innovation of having a single character generator provide the text font for both screen and copier.

The basic layout of the terminal, with the screen and main keyboard on the left and the blank area on the right, was intended to allow the system to be upgraded. The printer was one such upgrade, but over time DEC offered a number of other options. The large size of the cabinet was deliberate, to avoid a cooling fan. The two circuit boards with processor and memory at the base of the terminal, and a single board with power-supply and monitor electronics at the rear, were cooled by convection. The large, flat top of the terminal frequently accommodated large volumes of DEC documentation, which could block the vents and cause overheating.

VT50
The VT50 was the first terminal Digital produced in this cabinet. It provided only 12 lines of text with blank lines between them to use the entire vertical area of the display. Like its predecessor, the VT05, the VT50 did not support lowercase letters. Computer users of that era seldom needed lowercase text.

VT52
The VT50 was soon replaced by the upgraded VT52. The VT52 had considerably larger buffers, giving it the capacity to store not only a full 24 lines of text that better utilized the screen space, but also the text off the top and bottom of the screen. This allowed the terminal to scroll backwards a limited amount without having to ask the host to re-send data. The VT52 also included lowercase text support and a host of other new features. It was the first DEC terminal that allowed WYSIWYG text editing. The VT52 supported communications speeds of: 75,110,150,300,600,1200,2400,4800,9600 bps

VT55
The VT55 incorporated an add-on graphics system that was capable of displaying two mathematical functions or histograms. This was invoked by sending a command string that sent the terminal into graphics mode, with further data being sent to a separate buffer and CPU. Both systems mixed their data during the display, allowing the user to mix graphics and text on a single screen, as opposed to systems like the Tektronix 4010 or plotters that had to slowly draw text using graphics commands. This system became known as waveform graphics, and would re-appear on the later VT105.

VT61 and VT62
The VT61 and VT62 were block-mode terminals. The VT62 was to be used in conjunction with TRAX, a transaction processing operating system on high-end PDP-11's. They used the same cabinet but had a more complete custom processor. Application-specific behavior was coded in separate PROM memory, using a separate instruction code that the processor interpreted. This unpublished language was to be used to easily develop additional models specific to single Digital marketing organizations. These terminals synthesized a "tock" sound on a speaker for feedback when a key was pressed, whereas the VT5x activated a relay.