Raster Scan Monitors

Raster Scan Monitors

(Instructor’s notes)

Monochrome raster scan monitors

Sections:

CRT tube operation

CRT

Heater

Cathode

First anode

Screen grid

Focus grid

Last anode, high voltage

Phosphorus coating on screen

Electron beam

Deflection yokes

Show how a beam gets to the screen.

Show how signals through the deflection yokes draw the beam across the screen to create a raster.

Vertical oscillator

Vertical output stage

Vertical yoke

Horizontal oscillator

Horizontal Output Transistor

Horizontal yoke

Flyback transformer and high voltage section

Show how video signals on the cathode control the intensity of the beam to get a picture on the raster.

Show sync signals from the CPU that put the video information in sync with the raster to make a picture.

Color raster scan monitors:

Show differences in the CRT design.

Three guns, red blue, green.

Colored phosphors

Screen mask

Briefly mention convergence procedures and rings (preset at factory, don’t touch them).

Show how to change a tube.

Discharge the anode safely.

Never assume it’s completely discharged.

Touchscreens:

(See attached file)

Touch Screens

Overview

First, we will go over touch screen systems, in general terms. Them we will go into specific applications and devices.

Generally

The touch screen, itself is a sheet of plastic, or glass. One side is coated entirely with a very thin, transparent coating of conductive material. This is usually the inside (concave side). The player side (convex side) has similar coating in each corner. A signal (high audio range, usually) is sent through the concave side, and is coupled through the screen, to the convex side. These four channels feed to amplifiers and an Active Filter. The Band Pass output of the Active Filters picks off the signal, filtering out other noise. The four channels then get selected by a multiplexer. The selected channel is fed to an Analog to Digital Converter. A local microcomputer selects the channels, and reads the output of the ADC.

Calibration sets a nominal signal level being fed to the concave side by changing the amplitude of the signal. If the screen is not being touched, a predictable signal level is sensed by all four channels. When the screen is touched, the signal levels are disrupted. How much each channel is altered tells where on the screen the touch occurred.

The fact that the screen has been touched, and at what coordinates, is relayed to the system microprocessor by serial communications.

An X and Y coordinate is given telling the system where the screen had been touched.

Specific systems, IGT Touch Screen Games

(Ref. Schematic 754-239-10, page 1)

Note please, on the top left corner of the page, we have the connector going to the screen, itself. SCRN0 (Screen 0) goes to the concave side. SCRN1 goes to the pickup that extends about a third of the way up and across from the Lower Right corner of the convex side. Likewise, SCRN2, the Upper Right, SCRN3 the Lower Left, and SCRN4 the Upper Right.

The Front End Board (75423920) receives two Multiplex signals (MUX0 and MUX1) that select one of the four channels of the Multiplexer, three Attenuation signals (ATTN0, ATTN1, and ATTN2), and a 200 KHz signal from the Control board. The Front End Board return the selected Analog channel to the Control Board, and a 10 KHz timing signal.

The 200 KHz signal is divided down to 10 KHz by U8. This 10 KHz signal is fed back to the Control Board as a strobe that feeds to the INT1* line of the microcomputer, U15. This same 10 KHz signal is fed to U6 and U9B that make up an Attenuator Circuit. U6 selects one of eight resistors that controls the gain of U9B. ATTN0, ATTN1, and ATTN2 from the microcomputer control which resistor is to be selected. This is done during Calibration (while the message on the screen is telling you not to touch the screen).

The Calibration process sets up a predictable maximum (or minimum) signal that appears at the output of the Active Filters. This 10 KHz signal is fed to the concave side of the touch screen (SCRN0) through C29, and to the Differential Amplifiers for each of the four channels. Coming out of U9B we should have a bipolar signal of 10 KHz. It is worth noting that the falling edge of our 10 KHz signal coming out of U8B, pin 13, is fed to the active low edge trigger INT1* of U15. This triggers the microcomputer to read the channel (supposed). This falling edge is inverted by U9B and becomes a positive going pulse (at a 10 KHz rate) that goes through the touch screen and comes out as our 10 KHz signal, mentioned above.

TXD and RXD from U15 are our serial port going out to the game, through U3. COMM0 through COMM4 is the game interface signals. COMM0 is Master Reset from the game, and resets U15 when the game does a reset (active low on COMM0). COMM1 is Serial data out (from the TS to the game). COMM2 is +13V from the game, supplying power to the game side of U3. COMM3 is serial In (from the game to the TS). COMM4 is ground, from the game side.

U19 is the Analog to Digital Converter. Although the ADS774 is capable of 12 bit operation, we can see that it is being used in 8 bit Mode only because the 8*/12 control line (pin 2) is tied low. The lower for data lines (of the twelve possible) are tied to the upper four data lines, and only the upper eight data lines are used for ADC data. U14A (and address line A15 being low) does address selection for address 4000 (or 4xxx, actually), and the A0 line comes into AD0, pin 4, to give us the 4000 and 4001 addressing. The STAT (Status output) is high when the ADC is busy, and goes low when conversion is done. The low out of STAT goes to the INT0* line of U15, telling the processor to read the ADC port.

When the processor (U15) gets the 10 KHz timing pulse on INT1* it selects one of the four channels and starts the ADC. When the ADC is done it generates INT0* and the processor reads the ADC to get a value for that channel, selects the next channel, and this process continues until all four channels are read. It is assumed that the processor doesn't e Active Filters. When the microcomputer gets the strobe pulse it selects and reads all four channels. All four channels are read 10,000 times a second, during each of the positive going pulses going through the screen.

Each of the four channels from the Touch Screen come in through U5A, B, C and D. A Differential Amplifier with, non-inverting, with a gain of about 2. Coming out of the four sections of U5 we have not only our 10 KHz signal, but 60 Hz hum, noise, and maybe harmonics of our 10 KHz signal. U10 and U27 are each a four channel Active Filter. Each of the signals from the touch screen goes through two sections of these Active Filters. The first section inverts the signal and acts as a High Pass filter, getting rid of everything below 10 KHz. The second section inverts our signal back to a high going pulse coming out of the Band Pass section, so only our 10 KHz signal from the touch screen should be present. The amplitude of the signal will depend on where on the screen we are touching it. This same process is done on the other channels, so 10,000 times a second we have four pulses coming out of the Active Filter section. The amplitude of the four pulses depends on where the screen is being touched, if at all.

(CAUTION TO THE READER, THIS FILE HAS BEEN CORRUPTED BY MY VIRUS CHECKER, AND HASN’T BEEN REBUILT YET. PLEASE IGNORE THE FOLLOWING OBVIOUS ERROR.)

These four channels are fed to U7, a Multiplexer. We pick off two outputs from the first section. From the output ??? just 10 K, and the Low Pass output, getting rid of everything above 10 KHz, and Inverted by U9A, that should give a gain of about 2, inverting. After getting the 10 KHz timing pulse, the Control section reads each of the four channels to see if the screen is being touched.

The control section is mostly our old familiar friend, the 80C32 (MCS-51 family, enhanced internal registers, external program memory, CMOS version) running at 22 MHz. U12 latches the lower eight address lines from Port 0 at ALE time (standard). U11 is main program space (32K x 8) covering the lower address range of Program memory (0000 to 7FFF). Chip selection for U11 comes from A15 (the upper address line) being low and PSEN* (Program Strobe Enable) being low.

U16 is intended to be Flash PROM, writable, non-volatile memory. U16 can be referenced in two ways. It can be referenced as the upper half of program memory (8000 to FFFF) using A15 and PSEN, or it may be read or wrote to at these same addresses of data space.

U17 is an 8K x 8 CMOS RAM. Main Data memory, from 0000 to 3FFF of data space. U19 is the Analog to Digital section at address 4000 and 4001 of data space. U14A and B does address selection for these circuits.

U20, a MAX690, handles Power On Reset and Watchdog Timer functions (using Port 1, bit 7 of U15). No creative surprises here.

U18A and B picks off the 22 MHz oscillator signal from U15 and divides it down to 200 KHz, which will eventually be divided down by the front end section to be of data space.

U17 is an 8K x 8 CMOS RAM. Main Data memory, from 0000 to 3FFF of data space. U19 is the Analog to Digital section at address 4000 and 4001 of data space. U14A and B does address selection for these circuits.

U20, a MAX690, handles Power On Reset and Watchdog Timer functions (using Port 1, bit 7 of U15). No creative surprises here.

U18A and B picks off the 22 MHz oscillator signal from U15 and divides it down to 200 KHz, which will eventually be divided down by the front end section to be our 10 KHz signal, mentioned above.

TXD and RXD from U15 are our serial port going out to the game, through U3. COMM0 through COMM4 is the game interface signals. COMM0 is Master Reset from the game, and resets U15 when the game does a reset (active low on COMM0). COMM1 is Serial data out (from the TS to the game). COMM2 is +13V from the game, supplying power to the game side of U3. COMM3 is Serial In (from the game to the TS). COMM4 is ground, from the game side.

U19 is the Analog to Digital Converter. Although the ADS774 is capable of 12 bit operation, we can see that it is being used in 8 bit Mode only because the 8*/12 control line (pin 2) is tied low. The lower for data lines (of the twelve possible) are tied to the upper four data lines, and only the upper eight data lines are used for ADC data. U14A (and address line A15 being low) does address selection for address 4000 (or 4xxx, actually), and the A0 line comes into AD0, pin 4, to give us the 4000 and 4001 addressing. The STAT (Status output) is high when the ADC is busy, and goes low when conversion is done. The low out of STAT goes to the INT0* line of U15, telling the processor to read the ADC port.

When the processor (U15) gets the 10 KHz timing pulse on INT1* it selects one of the four channels and starts the ADC. When the ADC is done it generates INT0* and the processor reads the ADC to get a value for that channel, selects the next channel, and this process continues until all four channels are read. It is assumed that the processor doesn't confirm a valid touch on the screen until a few consecutive hits have been registered. It then sends info to the game regarding where, on a table of given X and Y coordinates, the touch has occurred.

Back on page one. U25 takes the +13V from the game power and converts it down to +7V for the front end analog power. U26 brings this down to +5V for the digital circuits. U23 converts the +13V to about -13V, and U24 brings this down to -7V for the front end analog section.

What usually goes wrong!!!

The most common failure I have seen is the touchscreen looses sensitivity at the corners. Buttons at the corners fail to work (SPIN). As we have seen in operation described above. The sense sections are located at the corners. As the finger draws nearer the corner, the amplitude increases. After time, temperature, or what ever condition it is, the signal loses amplitude, and can't get high enough to be recognized as being that close to the sensor. Recalibration becomes necessary. It seems as though calibration becomes harder when the game gets warm.