Chapter 7
VT52 Printer Interface Option

7.1 VTXX-KA Introduction

The VT52 Printer Interface Module, VTXX-KA, is a field-installable option available on all new VT52s, and certain units already in the field. It provides a data path between the VT52 video terminal and a parallel printer such as the LA180 or a serial printer such as the LA36 or the LA180 with a serial interface.

7.2 Printer Interface Block Diagram

Figure 7-1
VTXX-KA Block Diagram

Figure 7-1 shows the basic block diagram for the printer interface module and related circuits.

7.2.1 Parallel Operation

The VTXX-KA transfers ASCII data from the VT52 RAM via the data bus D00-D06 to the parallel interface port J5. Drivers E1-E4 are impedance matched to the LA180 input data lines and can drive the data up to 100 feet.

The demand line from the LA180 is raised when the LA180 is ready to accept a new character. This signal sets the flip-flop E10, which asserts the character flag signal (CCFL) to the VT52. On the next cycle of the VT52 microprogram the Enable Cycle signal (EN CYCLE L) is asserted and the VT52 places an ASCII character on the data bus. A Parallel Strobe signal (P STROBE L) is generated to clock the ASCII character to the LA180. At the same time the strobe resets flip-flop E10, thereby deasserting the character flag signal (CCFL) to the VT52. This completes a character transfer cycle. The cycle repeats itself each time the LA180 raises the Demand flag.

During this process the Paper Fault and ON LINE signals from the LA180 enable the character scan flag (CSFL) signal to the VT52. The process is interrupted by paper out, local mode or the cover interlock switch at the LA180. Additionally, the selector switch S1 (Paragraph 7.1) on the VTXX-KA module must be position 1 or the character scan flag (CSFL) signal will be disabled and no data transfer will occur.

7.2.2 Serial Operation

During the serial operation, the serial hardcopy device requests data by asserting DATA TERMINAL READY (DTR) through the serial interface. As long as this signal is asserted the VTXX-KA enables the Character Scan Flag (CSFL) signal to the VT52. The signal Transmitter Ready (XRDY) asserts the Character Flag (CCFL) signal to the VT52. On the next cycle of the VT52 microprogram, the Enable Cycle (EN CYCLE L) is used to generate a Parallel Strobe (P STROBE L) which clocks the ASCII data into the UART. The UART (E5) is used to convert parallel ASCII data from the VT52 to serial formatted data. As the data is clocked in, the UART deasserts Transmitter Ready (XRDY). The Transmitter Ready (XRDY) signal is reasserted after the UART has completed the serial character transmission.

NOTE
Jumpers (not labelled in the schematic) on the NB1, NP, EP and XD7 input to the UART, allow character parity selection, odd or even.

Serial data from the UART is placed on pin 3 of the serial EIA interface connector J6 after conversion from TTL to EIA by the EIA driver E12.

EIA outputs Clear to Send and Data Set Ready are held at 12 V whenever the VT52 is powered on. (The VTXX-KA draws its power [+5 V, -12 V, +15 V] from the VT52 power supply via the Data Paths, Memory and Decoders board).

Serial data transfer is enabled whenver Data Terminal Ready (DTR) is asserted. The functionality of DTR depends upon the printer and EIA device selected as follows:

LA35 or LA36 with the LAXX-KG (EIA data only) interface option:
DTR is asserted whenever printer power is ON. There is no prevention of data loss when the printer is disabled by a paper out, local mode or cover interlock switch.
LA35 or LA36 with the LAXX-LG (EIA with modem control) option:
DTR drops on local mode, paper out, cover interlock, or power down. The VT52 reverts to normal operation.
LA180 with the EIA Serial Interface LAXX-NW option:
XON/XOFF protocol selected on the LAXX-NW cannot be used to control data flow to the FIFO buffer since there is no transmit data line into the VTXX-KA.

DTR may be controlled by paper out or the contents of the FIFO buffer. The latter will have to be selected if the data throughput of the VT52/printer exceeds the print speed of the LA180.

7.3 VTXX-KA Installation

Figure 7-2
Module Interface Relationship

Figure 7-2 shows the interrelationship of the RUT module, the DP module, the VTXX-KA module, and the character generator module as well as the various communication interface connectors.

The installation of the VTXX-KA printer interface module consists of removing the base assembly and character generator module which is located on the RAM and Data Path module. The VTXX-KA module plugs into the DP module and is held in place by two screws which secure it to two threaded hex head stand-offs which are factory installed. The character generator module is then reinstalled on the VTXX-KA module and the interface cables connected to the VT52. All parts necessary to perform the VTXX-KA module installation are supplied with the module and are listed in Table 7-1.

Table 7-1 VTXX-KA Parts List
Part No, Description Quantity
54-12046
(MS40595)
VT52 Printer Interface Module 1
90-06036-01 #8-32 × 5/16 inch Screw 2
90-06040-1 #8-32 × 5/8 inch Screw 1
90-08072 #8 Lockwasher 2
90-07081 ¼ inch Cable Clamp 1
90-07084 7/16 inch Cable Clamp 1
90-07016 Grommet 1

7.3.1 Installation of the VTXX-KA Module

Perform the following steps to install the VTXX-KA printed interface module.

  1. Make certain that the terminal line cord is not plugged into an electrical outlet and that the power (ON/OFF) switch is in the (OFF) position.
  2. Disconnect the VT52 interface cable from the host computer.
  3. Place the unit upside down on a bench or table with the CRT facing you (Figure 7-3).
  4. Remove the base (Paragraph 5.3).
  5. Remove the character generator from the RAM and Data Path module by alternatively lifting the left and right edges of the character generator module until both 11-pin module connectors are free. Do not bend the pins or place excess stress on the module (Figure 7-4).
  6. The VTXX-KA module plugs into J5 and J6 on the DP module (Figure 7-2). To install the VTXX-KA module align the two 11-pin male connectors (J2 and J4) with the two 11-pin female connectors (J5 and J6) on the DP module and alternately apply pressure to the left and right VTXX-KA connectors until they properly seat (Figure 7-5).

    NOTE
    Be sure not to bend the pins and check to see that the VTXX-KA connector pins are entering the proper holes on the DP module connectors. The cable connectors on the VTXX-KA module must face the front of the VT52.

  7. Secure the VTXX-KA module to the two threaded hex standoffs on the DP module using the two 8-32 × 5/16 screws and #8 lockwashers (Figure 7-5).

    NOTE
    The VTXX-KA module may have to be moved slightly to align the mounting holes with the threaded hex stand-offs on the DP module. Do not overtighten the two mounting screws.

  8. Install the character generator by aligning the two 11-pin male connectors (J1 and J2) on the character generator with the two 11-pin female connectors (J1 and J3) on the VTXX-KA module. Alternately apply pressure on the left and right edges of the character generator until properly seated (Figure 7-2).

    NOTE
    The arrow and the word "REAR" on the etch side of the character generator must be positioned on the left side of the VTXX-KA module. The arrow points to the rear of the VT52 when the character generator is properly installed (Figure 7-6).

    Figure 7-7
    VTXX-KA Printer Interface Module
  9. Connect P2 from the VTXX-KA module to J4 on the DP module.
  10. Connect P1 from the VTXX-KA module to J6 on the RUT module. When properly installed the black cable wire from P1 will be towards the right of the VT52 (Figures 7-2 and 7-7).
  11. Select the desired interface mode (serial or parallel) and the desired baud rate by turning the VTXX-KA baud rate switch (Figure 7-7) to the appropriate position. Paragraph 7.5 describes the various switch selections.
  12. Perform the appropriate cable installation procedure described in Paragraph 7.3.2.

7.3.2 VTXX-KA Cable Installation

In addition to the interface cable from the VT52 to the host computer there is an interface cable required between the VTXX-KA module and the user's printer. This cable may be an EIA type cable for a serial interface or may be a parallel interface cable for a parallel interface. Paragraph 7.3.2.1 describes the installation procedure for a serial interface cable while Paragraph 7.3.2.2 defines the procedure for installing a parallel interface cable.

7.3.2.1 Serial EIA Printer Interface Cable Installation - Figure 7-8 shows a typical EIA installation and Table 7-2 shows J6 EIA signal locations.

  1. Remove the grommet from the host computer interface cable.
  2. Mate the male connector from the serial interface cable to the female serial port J6 on the VTXX-KA module (Figure 7-7).

    NOTE
    The two hold-down screws on the printer interface connector (some connectors do not have these screws) cannot be secured to the VTXX-KA module.

  3. Pass the host computer and printer interface cables, as well as the AC power cord, through the VT52 base (Figure 7-9).
  4. Position the base on the VT52 main assembly and tilt the base towards the rear of the unit.
  5. Using a ¼ inch nut driver, secure the ground strap from the main assembly to the base.
  6. Lower the base onto the VT52 main assembly and turn the five base fasteners ¼ turn clockwise with a ¼ inch nut driver.
  7. Place the grommet from the VTXX-KA kit around the host computer and printer interface cables. Insert the grommet into the VT52 base plate (Figure 7-9).
  8. Replace the AC power cord access cover securing it in position with one screw towards the rear of the terminal.
  9. Place the ¼ inch cable clamp from the VTXX-KA kit around the serial interface cable. Secure both the host computer and printer interface cables to the VT52 base with the 8-32 × 5/8 screw supplied with the VTXX-KA kit (Figure 7-9).
  10. Replace the base front screen on the VT52 base securing the three screws with a ¼ inch nut driver.
  11. Return the VT52 to its normal operating position.
  12. Connect the serial printer interface cable to the printer.
  13. Connect the host computer interface cable to the computer or modem with which you wish to communicate.
  14. Connect the AC power cord to an electrical outlet that meets the requirements of the terminal.
  15. The installation of the VTXX-KA is now complete. Perform the initial checkout procedure given in Paragraph 7.5.

    NOTE
    The 7/16 inch cable clamp (Pin 90-7084) from the VTXX-KA kit is not used with the serial interface cable.

Table 7-2 J6 EIA Signal Locations
VTXX-KA
Pin Number
EIA Circuit
Designation
Description
1 AA Protective Ground
2 BA Transmitted Data
3 BB Received Data
4 CA Request to Send
5 CB Clear to Send
6 CC Data Set Ready
7 AB Signal Ground
20 CD Data Terminal Ready
* All pins not listed above are not used

7.3.2.2 Parallel Printer Interface Cable Installation - Figure 7-10 shows a typical parallel installation. Table 7-3 gives the VTXX-KA parallel port (J5) signal locations.

  1. Remove the communication interface adapter located on the RUT module (Figure 7-4).
  2. Pass the parallel interface cable through the AC power cord access hole in the VT52 base. This is accomplished by folding the connector up against the cable to pass through the access hole. Unfold the cable after passing it through the access hole.
  3. Connect the 40-pin female connector from the parallel interface cable to the 40-pin male connector J5 on the VTXX-KA module (Figures 7-2 and 7-7).
  4. Pass the ac power cord through the ac power cord access hole in the VT52 base.
  5. Position the base on the VT52 and tilt it towards the rear of the unit.
  6. Secure the ground strap from the VT52 main assembly to the base using a ¼ inch nut driver.
  7. Lower the base onto the VT52 main assembly and then turn the five base fasteners ¼ turn clockwise with a ¼ inch nut driver.
  8. Place the 7/16 inch cable clamp from the VTXX-KA kit around the parallel printer interface cable. Secure the cable clamp to base using the screw hole to the rear of the ac power cord access hole and the 8-32 × 5/8 inch screw from the VTXX-KA kit (Figure 7-11).

    NOTE
    The cable access hole cover which was removed during disassembly is not reinstalled when the round parallel interface cable is used.

  9. Reinstall the communications interface adapter onto the RUT module with the host computer interface cable towards the left of the VT52 (Figure 7-4).
  10. Adjust the slack in the parallel interface cable so that the cable does not rest on top of the communication interface adapter.
  11. Insert the host computer interface cable (along with the grommet) into the slot in the VT52 base plate (Figure 7-11).
  12. Secure the host computer interface cable strain relief to the VT52 base plate.
  13. Replace the base front screen securing the three screws with a ¼ inch nut driver.
  14. Return the VT52 to its normal operating position.
  15. Connect the parallel printer interface cable to the printer.
  16. Connect the host computer interface cable to the computer or modem with which you wish to communicate.
  17. Plug the ac power cord into an electrical receptacle that meets the power requirements of the terminal.
  18. The installation of VTXX-KA printer interface module is complete. Perform the initial checkout procedure given in Paragraph 7.5.

    NOTE
    The grommet (pin 90-07016) and the ¼ inch cable clamp (pin 90-07081) from the VTXX-KA kit are not used when the parallel interface cable is installed.

Table 7-3 J5 Parallel Port Signal Locations
Pin Number Signal Corresponding Pin on LA180 Printer
VV Data Strobe A
TT Data 5 C
RR Data 6 E
LL Data 2 K
JJ Data 1 M
FF Data 4 P
DD Data 7 S
BB Data 3 U
D Data 8 (parity) SS
X Demand Y
R Paper Fault EE
N On Line HH
UU, SS, PP, MM, KK, HH, EE, CC, AA, Y, W, U, S, P, M, H, E, C, A Ground B, D, F, J, N, R, T, V, X, Z, BB, DD, FF, JJ, NN, RR, TT, VV
NN, Z, V, T, L, K, J, F, B Open H, W, AA, CC, KK, LL, MM, PP, UU

7.4 Initial Set Up

On the VTXX-KA interface module there is a small 8-position rotary switch (Figure 7-7). This switch selects which interface (parallel or serial) will be used and, if the serial interface is selected which baud rate will be used.

NOTE
The VTXX-KA baud rate is independent of the VT52 on-line baud rate but must match the baud rate of the printer.

The switch has the following positions:

1 parallel
2 serial 150 Baud
3 serial 300 Baud
4 serial 600 Baud
5 serial 1200 Baud
6 serial 2400 Baud
7 serial 4800 Baud
8 serial 9600 Baud

Position 1 is the most counterclockwise position. To select a certain position, turn the switch fully counterclockwise, then count to the desired setting. If there is no printer attached to the VT52, the selector switch should be set to one of the serial positions to ensure that the terminal does not receive erroneous signals from the VTXX-KA parallel circuitry.

7.5 Initial Test Procedure

The initial test procedure verifies proper operation of the VT52 after installing the VTXX-KA printer interface.

Perform the following steps:

  1. Plug the line cord of the VT52 into an electrical outlet that meets the requirements of the terminal.
  2. Apply power to the unit by setting the power (ON/OFF) switch to ON. This switch is located on the right side of the terminal.
  3. Place the printer on-line and be sure it has a supply of paper.
  4. On the VT52, press the copy key. The key should click when pressed, and the printer paper should advance 24 lines.
  5. Place the VT52 in the local mode.
  6. Type your name for several lines on the VT52 screen. Press the VT52 copy key. Several lines of print with your name should be printed as they appear on the screen.

If after performing the initial checkout procedure the printer does not output the proper results, refer to Paragraph 7.8 for troubleshooting information.

7.6 VTXX-KA Operation Instructions

When the VTXX-KA module is installed, the operator can make the VT52 output data to the printer by using the copy key on the VT52 main keyboard. There are two variations of the output to the printer depending on whether the operator holds the shift key down when the copy key is typed.

If the copy key is typed without the shift key down, then the terminal will output the entire contents of the screen to the printer.

If the copy key is typed while the shift key is held down, then the terminal enters Auto Print mode. While in the Auto Print mode the terminal can output a variable number of lines of text to the printer. Whenever the terminal gets a line feed command to move the cursor down one line, the line that the cursor was on at the time will be output to the printer. This will be true as long as the terminal is in the Auto Print mode. To take the terminal out of Auto Print mode, the operator types the copy key again while holding down the shift key. None of these three uses of the copy key cause any codes to be sent to the host.

7.7 VTXX-KA Programming Information

7.7.1 Host Computer Commands

The three commands which the operator can issue by using the copy key can also be issued from the host computer. To command the terminal to send text to the printer, the host sends the VT52 two character commands called Escape Sequences. A separate Escape Sequence exists for each of the three functions:

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

There are two additional ways in which the host computer can command the terminal to send text to the printer: by specifying individual lines to be printed; or by placing the terminal in a mode in which it sends the received characters to the printer instead of to the screen (Printer-Controller Mode). The Escape Sequences for the two additional print modes are:

ESC V Print Cursor Line (033 126)
ESC W Enter Printer-Controller Mode (033 127)
ESC X Exit Printer-Controller Mode (033 130)

When the terminal receives ESC V from the host, it will output the entire contents of the line which the cursor is on to the printer. An ESC W sent from the host to the terminal causes the VT52 to enter Printer-Controller Mode. During the Printer-Controller Mode, every character the VT52 receives will be passed directly to the printer without being acted upon by the VT52. The VT52 becomes essentially a remote printer controller. ESC X will cause the terminal to exit Printer-Controller Mode.

NOTE
The characters ESC W cause the VT52 to enter the Printer-Controller Mode and are not sent to the printer. When the VT52 receives ESC X, which causes it to exit Printer-Controller Mode, the ESC will have already been sent to the printer. Instead of sending the X to the printer, the VT52 sends the character CAN (whose code is 030), which cancels the effect of the ESC.

7.7.2 Data Format

In the previous paragraph, four print commands have been described. Each of these commands cause a certain number of lines to be printed. The print line command causes one line to be sent to the printer and the print screen command causes 24 lines to be printed. Although entering or exiting Auto Print Mode does not cause anything to be printed, 1 line is sent to the printer each time a LF is received when the terminal is in Auto Print Mode. Each of these lines must first appear on the video display of the VT52.

A line on the VT52's display consists of 80 character positions. In each character position there may be a space or a displayable character. The text which appears on a given line may not use all 80 character positions. In this case, the spaces will remain in the character positions to the right of the text.

When the VT52 sends a line to the printer, it first sends the ASCII code for the character in the leftmost column (column 1) to the printer interface. Then it sends the ASCII code for the character in column 2, and so on. After it has sent the code for the character in column 80, the VT52 sends the code CR (015), and then LF (012) to the printer. These last two codes cause the printer to return to column 1 of the next line to the paper.

The terminal will ensure that the text on the line is not changed between the time the print command is issued and the time the characters are physically sent to the printer by using the XON and XOFF signals to the host.

If there are spaces in a line of text, the VT52 sends the code for Space (040), to the printer to mark the position where the spaces occurred in the screen. This causes the printer to advance the printhead so that a space is actually seen at the correct position. However, if spaces occur past the end of the text, there is no point in sending those spaces to the printer. Therefore, to save time, if 16 or more spaces occur to the right of the text on a line to be printed, the VT52 will trim all spaces occurring after the last printable character by sending a CR and an LF to the printer.

NOTE
If the printer has more than 80 columns, the columns to the right of the 80th column will never be used by the single line print commands since the VT52 has only 80 columns. However, many programs will want to control the printer in more detail (print in 132 columns, overprint lines, send control codes to the printer etc.), without the interpretive action of the VT52 (trimming blanks and inserting line delimiters). This is accomplished by placing the terminal in the Printer-Controller mode. The VT52 then passes every character it receives directly to the printer without converting or otherwise acting on the characters. The VT52 becomes essentially a remote printer controller.

7.7.3 Synchronization

Although the action of the printer is not instantaneous and may lag behind data transmission from the host to the terminal, provisions exist to ensure that all requested output goes through to the printer. In three situations where requested information to be printed might be destroyed or altered before it can be printed, the terminal will cease processing incoming characters:

  1. In Auto Print Mode, when the cursor is moved up to a line that was marked for printing but has not yet been printed, the terminal will cease processing incoming characters until the cursor line is printed.
  2. In Auto Print Mode, if the top line was supposed to be printed but has not yet been printed, 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 printed. (This is the only situation in Auto Print Mode in which scrolls are prohibited. In general, the terminal does not require that data being printed be on the same line on the screen that it was when the request was made to print it).
  3. The terminal will not process incoming characters after a Print Screen or Print Line command, until the specified data has been printed.

In these situations, the terminal begins putting incoming characters in its silo buffer, and transmits XOFF to the host. When the necessary printing 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.

When the terminal is in Printer Controller Mode, so that data flows through the terminal and out to the printer as it is received from the host, the terminal will use the silo, and XOFF and XON signals, to limit the data rate on the transmission line to a rate the printer can keep up with. Therefore, the maximum data rate for output to the printer via Printer Controller Mode is the minimum of the baud rate between the terminal and the printer, and the baud rate between the terminal and host. For example, if the terminal were set to communicate with the host at 9600 baud, and to communicate with the printer at 1200 baud, then the terminal would use the XOFF and XON signals to limit the net data flow to 1200 baud. If the terminal were set to communicate to the host at 300 baud and with the printer at 1200 baud, the host would never receive an XOFF from Printer Controller Mode. The transmission would be limited to 300 baud by the line between terminal and host.

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 (must suspend its transmission promptly upon receiving XOFF, and resume upon receiving XON), or else the silo may overflow, interfering with the accuracy of the output.

Specifications for exactly how promptly the host must respond to XOFF by ceasing output are found in the DECscope User's Manual.

NOTE
In the situations listed above where the terminal refuses to process characters from the host, the terminal will still allow the operator to use the keyboard to transmit characters 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 printer is running, since he could cause the silo to overflow by his typing.

7.7.4 Status Reporting

Whenever any VT50-series terminal receives the Escape Sequences ESC Z, it transmits a three-character identification sequence to the computer which uniquely specifies its model number and capabilities, so that software can determine what features are available in the terminal. This identification sequence will allow the computer to determine whether a printer is available on the terminal.

ESC / K VT52 without printer
ESC / M VT52 with printer

The terminal will transmit the second Escape Sequence only if it determines that the printer is capable of printing.

If the Selector is set to one of the serial positions, then the VT52 will respond to the computer with ESC / K (no printer) if the signal DTR (Data Terminal Ready) is deasserted (Paragraph 7.2.2).

If the Selector is set for parallel transmission, then the VT52 will transmit the "no printer" Escape Sequence if either the ON-LINE signal is deasserted, or the PAPER FAULT signal is asserted (Paragraph 7.2.1).

When no printer is plugged into either the serial or the parallel slot, the Selector should be set to one of the serial positions. Doing this will ensure that the terminal will always report to the computer that a printer is not present.

7.8 VTXX-KA Troubleshooting

If after performing the initial checkout procedure the printer does not print the correct results, do the following:

  1. Make sure the printer power is on, the printer is on-line and that the printer has a supply of paper.
  2. Check the printer interface connections at the printer and the VT52.
  3. If the printer is a parallel printer, check the baud rate switch on the VTXX-KA module. It should be set to position 1 for parallel operation.
  4. If the printer is a serial printer and the message is unintelligible, check the baud rate switch on the VTXX-KA module. It should match the baud rate of the printer.
  5. Type ESC Z on the VT52. The character M should be displayed on the screen. If the character K is displayed recheck the printer for an adequate paper supply and on-line operation. If the character M is displayed but the printer will not print any data from the VT52 further testing is required.

NOTE
If the VTXX-KA fails to operate, ensure the RUT module contains the proper PROM/ROM variation (Appendix A).