Chapter 6
VT50-Series Copier Option

6.1 VT50-Series Copier Introduction

The electrolytic copier provides the video terminal with hard copy capability. The copier can print a screenful of information or can print the information line-by-line.

6.2 Copier Block Diagram

Figure 6-1
Copier Block Diagram

Figure 6-1 shows the basic block diagram for the copier and related circuits. When the keyboard COPY key is pressed, the microprogram issues the COPY ENABLE L (CEN L) command to the copier logic via the connector board. If the anode cover switch is closed, relay K1 picks up, and AC power is applied through the triac to the helix and paper feed motors.

As the helix rotates, the microprogram of the VT52 waits for the COPY SCAN FLAG (CFS L), which indicates the helix is at left margin. The first COPY LOAD L (CLD L) signal from the VT52 resets the scan flag.

As the helix completes one rotation, 80 COPY CHARACTER FLAGs (CCF L) are generated by the logic. On each flag the VT52 responds by sending the data output from its character generator (CD0-CD7) along with a COPY LOAD L pulse. CLD L clocks the data into the data buffer on the connector card and resets the copy character flag.

The timing and control section of the logic transfers the contents of the data buffer to the S/R and then shifts the data out, serially, to the helix driver circuitry.

An OCI optically couples the data into the first stage of the helix driver. The -150 Vdc regulator provides a current source to the helix. The driver provides a 90 V pulse to the helix each time a metallic dot (DOT L) is to be provided to the paper.

Should a paper out condition occur, the anode would short directly to the helix, providing a ground to the paper out sensor. The paper out sensor de-energizes relay K1 which disconnects the helix from its source and turns off the triac. The motors stop.

A current sense resistor in the helix driver emitter circuit is used to detect a dry paper condition. As the paper dries, its conductivity decreases as does the anode-helix current each time a dot is generated. The OCI detects this condition and activates signal INHIBIT. INHIBIT prevents the generation of the CHARACTER FLAG to the VT52. At the same time, it is ANDed with the SCAN FLAG to produce a dot once each helix rotation. The result is a vertical line down the margin of the paper.

6.3 Copier Timing

Figure 6-2
VT Copier Timing

Figure 6-2 shows the copier timing sequence.

Character print timing within the VT52 hardcopy unit is generated from an optoelectronic encoder disk mounted on the helix motor shaft. One complete revolution of the disk corresponds to one line scan on the paper. The encoder has two tracks - an inner scan track consisting of a single aperture and an outer clock track with 81 suits.

When the VT52 is ready to transfer print data to the copier, it sets the COPY ENABLE signal (CEN L) low. The helix motor turns on and the encoder begins to turn. The VT52 waits for the COPY SCAN FLAG (CSF L) to go low. It responds with the first of 81 COPY LOAD (CLD L) pulses. This pulse resets the scan flag and enables the copier logic to develop COPY CHARACTER FLAGs from the clock track. As each character flag goes low the VT52 transfers a 7-bit dot pattern output from its character generator to the copier logic. The CLD L signal clocks the data in and resets the character flag.

A quick check to determine if the VT52 and copier are communicating is to check the character flag signal. If the signal remains low, and helix is rotating, problem is in VT52. If signal remains high, check scan flag. If scan flag remains high, the problem is in the copier.

6.4 Operating the Copier

When the COPY key is pressed, all the information currently being displayed on the screen is copied.

In addition to this screen-by-screen method of copying, a continuous-copying mode exists. Auto-Copy mode, like Hold-Screen mode, requires one command to enter it and another to exit it.

In Auto-Copy mode, downward movement of the cursor causes copying to occur. When a Line Feed or Cursor Down command is performed, the line the cursor was on is copied. If a Direct Cursor Addressing command moves the cursor from line 4 to line 9, then lines 4 through 8 are copied. (It is more accurate to say that such lines are not copied but "marked for copying", since the cursor can be moved down again, marking another line for copying, while previous lines are still being copied). The copier is activated whenever one or more lines have been marked for copying. If Auto-Copy mode is exited before all the marked lines have been copied, they will still be copied. To enter Auto-Copy mode, the operator holds the SHIFT key down and presses the COPY key. Doing so again will cause the terminal to exit Auto-Copy mode. Entering and exiting Auto-Copy mode does not mark any lines for copying.

To remove output from the copier, lift the metallic lever on the side of the unit where the paper appears -- the detent lever -- and pull on the paper until all the information is visible. Lower the detent lever, and pull the paper up sharply so that it tears on the serrated cutting edge built into the shell of the terminal. If the copier has been idle for several minutes, lift the detent lever and pull the paper out until the wet portion is reached before trying to copy anything. When the copier will not be used for an extended period of time, lift the detent lever to disengage the rubber rollers which feed the paper through the copier.

6.5 Programming the Copier

The COPY key performs three functions: when unshifted, it causes the screen to be copied; when shifted, it tells the terminal to enter or exit Auto-Copy mode. Software can perform these functions also, by transmitting Escape Sequences to the terminal. A separate Escape Sequence exists for each of the three functions.

Copier Commands Octal Codes
ESC ] Copy Screen (033 135)
ESC ^ Enter Auto Copy Mode (033 136)
ESC _ Exit Auto Copy Mode (033 137)

Here are two examples of software use of Auto-Copy mode:

  1. To output a file to the copier, software sends the following:
    1. CR and LF, if necessary, to ensure that the file begins on a new line.
    2. ESC J, if necessary, to erase extraneous data from the portion of the screen below cursor.
    3. ESC ^ to enter Auto-Copy mode.
    4. The body of the file.
    5. An LF must be supplied if it is not the last character in the file, in order to mark the last line in the file for copying.
    6. ESC _ to exit Auto-Copy mode.

    The file, and only the file, will be copied.

  2. To make a copy of the cursor line, the host sends the following commands: Enter Auto-Copy mode, Line Feed, Exit Auto-Copy mode, Cursor Up (seven characters). The cursor will be returned to its former location in the data, but if it was on the bottom line to begin with, this procedure will cause a scroll.

Although the action of the copier is not instantaneous and may lag behind data transmission, provisions exist to ensure that requested output is copied as it appears on the screen at the time of the copying request. In three situations where requested information to be copied might be destroyed or altered before it can be copied, the terminal will cease processing incoming characters:

  1. In Auto-Copy mode, when the cursor is moved up to a line that was marked for copying, but has not yet been copied, the terminal will cease processing incoming characters until the cursor line is copied.
  2. In Auto-Copy mode, if the top line was supposed to be copied but has not yet been copied, and if the cursor is on the bottom line and LF is received -- so that a scroll would be performed and the top line would leave the screen -- the terminal will not process the LF and subsequent characters until the top line has been copied. (This is the only situation in Auto-Copy mode in which scrolls are prohibited. In general, the terminal does not require that data being copied be on the same line on the screen that it was when the request was made to copy it).
  3. The terminal will not process incoming characters after a Copy Screen command, until the entire screen has been copied.

In these situations, the terminal begins putting incoming characters in its silo buffer, and transmits XOFF to the host. When the necessary copying has been finished, the terminal will begin to process the characters in the silo. When it has processed all the characters in the silo, the terminal will send XON to the host to indicate that it is ready for more characters.

Note that the host cannot determine if the terminal operator has pressed the COPY key. If there is any possibility that one of the above situations may occur, host software must support synchronization with XOFF/XON or the silo may overflow, interfering with the accuracy of the output.

In the three situations described above, characters typed by the operator are still transmitted to the host. If the terminal is switched for either off-line or local-copy operation, these characters will also go into the silo. Therefore, the operator of such a terminal should refrain from typing when the copier is running, since he could cause the silo to overflow.

6.6 Copier Removal and Replacement Procedure

  1. Remove ac power cord. The copier unit is installed in an area where dangerous voltages are present when the power cord is installed.
  2. Remove the copier cover.
  3. Remove the five screws that hold the copier to the shell. Gently lay the copier on a table or shell to facilitate detaching connecting cables.
  4. Detach the Mate-N-Lok connector that connects the copier to the ac line voltage and transformer secondary. It has four wires -- red, red/yellow, black, and white.
  5. Gently remove copier signal connector J1. A slight rocking motion may be used to accomplish this. Avoid breaking or bending the connector pins. Holding the data path module from the bottom will keep the mating socket ready while J1 is being retracted.
  6. Remove the green copier ground wire from the ground lug on the inside of the transformer mounting plate assembly. A nutdriver should be utilized or remove the fastening nut.
  7. Install the new copier by reversing the above steps.

6.7 Copier Maintenance

There are four maintenance functions the copier user must perform: changing the paper, replacing the anode (the printing blade), moistening the wick, and cleaning the helix wires. Doing all these things at the same time saves time and maintains the highest quality of copier output.

When the blue line appears on the side of the paper, there are only 2 to 3 meters left in the roll. Since the last meter of paper will be too tightly wrinkled to copy well, remove the paper and perform the maintenance at this time.

Access to the copier compartment is gained by lifting off the cover, taking hold at the serrated edge. Lift the print head assembly up by the three thumbscrews and swing it out toward you.

The anode is a long metal strip bent lengthwise. It is held in place by the thumbscrews. Turn the thumbscrews counterclockwise as far as they will go to release the anode. The anode can then be slid to the right and removed. Do not use your fingers to remove the anode! A worn anode often has a sharp edge where it has been rubbing against the paper. Instead, use a pencil to slide the anode out, by inserting it through one of the anode's windows.

Remove the paper cover, take out the remaining paper and the plastic hubs. Removing the paper will reveal a large cylinder with spiraled (helical) wires on its surface. This cylinder rotates when the copier is running. Salt deposits on the helix wires from the special copy paper used can cause streaks on the paper where characters are faint or do not appear at all. The helix wires can be cleaned with a damp cloth. Rotate the cylinder by hand in order to clean the entire surface. Do not try to remove the cylinder.

The wick pad and its plastic housing are in the paper chamber. Take hold of the housing by the round finger-hole and pull it out. Hold it under a faucet and run warm water over the felt, so that the wick is moistened. This helps to keep the copier paper moist throughout the life of the roll. By removing the housing to moisten the wick, the possibility of water leaking into the interior of the terminal is avoided. When the wick is moist, shake off the excess water and put the housing back in the paper chamber. Three tabs in the frame at the rear of the paper chamber will hold down the back of the wick housing. By applying pressure to the center of the housing, three tabs in the front of the paper chamber will come through rectangular holes in the wick housing, snapping the housing into place.

To install a new roll of paper, remove it from its plastic bag and unravel about ½ meter of paper. With one hand, lift the drag bar which is inside the paper chamber. With the other hand, place the roll in the chamber so that the paper issues from the underside of the roll. Thread the paper through the rollers, and out the side of the terminal, and replace the paper cover.

A new anode can now be slid in from the right. A new anode will not be dangerously sharp. Push it to the left as far as it will go and retighten all three thumbscrews when it is in place.

To summarize the six steps in this maintenance routine:

  1. Remove the anode.
  2. Remove the old roll of paper.
  3. Clean the helix wires.
  4. Moisten the wick.
  5. Install a new roll of paper.
  6. Install a new anode.

If the paper is allowed to dry, it will not work in the copier. Do not leave a roll of paper in the copier for more than a month, or in its plastic bag for more than six months. To avoid unnecessary wear to the helix wires, never operate the copier without paper.

The Character-Width lever is the only operator control found in the copier compartment. It is a lever located to the left of the paper chamber. This lever can be adjusted to produce characters of the desired width. Characters in the copier output which run together, or whose rightmost portions appear to be cut off, are indications that the character-width lever is set too wide.

6.8 Copier Troubleshooting

The troubleshooting chart in Table 6-1 lists the common trouble symptoms that could be observed during initial checkout or normal operation.

Table 6-1 Copier Troubleshooting Procedures
Symptom Probable Cause Solution
Copier fails to start when COPY key is pressed Paper empty Install paper.
Copier not receiving CEN L signal from VT Check for signal at J1, pin 10 (copier connector). Signal should go low when COPY is pressed. If not, VT is defective.
Defective relay K1 or defective triac Replace copier.
Defective cover switch Replace copier.
Copier starts up, but does not print Dry paper Check paper moisture content. Replace paper and dampen wick if necessary.
Fuse F1 blown Replace fuse on copier logic board (½ A)
Copier not receiving CLD L signal from VT Check CLD L signal at J1, pin 9. If signal is not present when copier motor is running, check CSF L at J1, pin 1. If signal remains low problem is in VT.
Copier prints a vertical line down margin Dry paper Dampen felt wick. Clean helix wire.
Copier image differs from screen image Copier defective Replace copier.
Paper Feed Paper improperly threaded Rethread paper.
Bad anode. Anode dragging on paper Anode dimension should not exceed 14.00 mm along the entire length.
Roller alignment Check drive and pinch roller for parallelism in vertical plane.
Humidor cover improperly seated Reseat cover so that mylar seal is not causing drag.
Felt insert improperly seated Snap felt insert into place.
Drag roller jammed Unjam roller.
Dirty drive and/or pinch roll Clean rollers of all oil or deposits.
Wrong paper Check that paper has the letter "K" prefixed to the lot number printed on the side.
Print character height variation Bad Anode Check Anode dimension.

6.9 Copier Specifications

Device Type Electrolytic copier
Principle Facsimile technology
Print Characteristics
Character Set Same characters as are displayable on video screen
Character Format Same dot matrix as video screen
Columns 80
Lines Variable number (continuous form)
Page Size There is no fixed page size, since the paper is unperforated and copying can take place on a line-at-a-time basis. Pages with the same number of lines as the video screen can be copied by a single command.
Copy Speed 30 lines per minute
Modes of Operation
  1. Page-by-Page Output -- The contents of the screen are copied on command.
  2. Auto-Copy Mode -- In Auto-Copy mode, each time an LF (Line Feed) is received, the line the cursor was on is copied.
  1. Operator Control -- The three copier commands -- Copy Screen, Enter Auto-Copy Mode, and Exit Auto-Copy Mode, can be issued from the terminal by use of the COPY key.
  2. Software Control -- The same commands can be issued from the host computer, in the form of Escape Sequences.
Synchronization The terminal transmits XOFF (or DC3; 0238) and XON (or DC1; 0218) to the host to regulate the flow of data from the host to the terminal during a copying operation. The terminal will accept and display data from the host during a copying operation unless a line that has yet to be copied would be modified. The terminal features a finite buffering capacity to prevent loss of data after XOFF is sent.
Operator Controls
Physical Feed lever (paper release)
Electronic Character width lever
Keyboard copy commands
Interface Internal to terminal. Terminal has EIA and current-loop connector options.
Spare Parts
Paper Electrolytic ("wet") paper, type K. Alfax paper is recommended; equivalent types are acceptable (available through DIGITAL).
Anode Standard DIGITAL part (normally included with electrolytic paper).
Routine Maintenance Required Changing paper, replacing anodes, cleaning helix, moistening wick

Copier environmental requirements are more stringent than those of the terminal.

Temperature 16°C to 32°C (59°F to 90°F)
Relative Humidity 20% to 80%

6.10 Related Documentation

The following list of documents contains useful information for installing, operating, and maintaining the VT50 series copier.

Document Title Document Number
VT50-Series Copier User's Manual EK-VT5C-OP-001
VT50-Series Copier Illustrated Parts Breakdown EK-VT50-IP-003

All purchase orders, invoicing inquiries, and correspondence should be forwarded to:

Digital Equipment Corporation
Cotton Road
Nashua, New Hampshire 03060
Attention: Accessory and Supplies Group