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Theory of Operation Teledyne API - Model 200EH/EM Operation Manual
226
KEYPAD DECODER
Each key on the front panel communicates with a decoder IC via a separate analog line. When a key is
depressed the decoder chip notices the change of state of the associated signal; latches and holds the state of
all eight lines (in effect creating an 8-bit data word); alerts the key-depress-detect circuit (a flip-flop IC);
translates the 8-bit word into serial data and; sends this to the I
2
C interface chip.
KEY-DEPRESS-DETECT CIRCUIT
This circuit flips the state of one of the inputs to the I
2
C interface chip causing it to send an interrupt signal to the
CPU
I
2
C INTERFACE CHIP
This IC performs several functions:
Using a dedicated digital status bit, it sends an interrupt signal alerting the CPU that new data from the
keyboard is ready to send.
Upon acknowledgement by the CPU that it has received the new keyboard data, the I
2
C interface chip
resets the key-depress-detect flip-flop.
In response to commands from the CPU, it turns the front panel status LEDs on and off and activates
the beeper.
Informs the CPU when the optional maintenance and second language switches have been opened or
closed (see Chapter 5 for information on these options).
DISPLAY DATA DECODER
This decoder translates the serial data sent by the CPU (in TTY format) into a bitmapped image which is sent
over a parallel data bus to the display.
DISPLAY CONTROLLER
This circuit manages the interactions between the display data decoder and the display itself. It generates a
clock pulse that keeps the two devices synchronized. It can also, in response to commands from the CPU turn
off and/or reset the display.
Additionally, for analyzers with the optional maintenance switch is installed (See Chapter 5), the display
controller turns on an LED located on the back of the keyboard interface PCA whenever the instrument is placed
in maintenance mode.
DISPLAY POWER WATCHDOG
The Model 200EH/EM’s display can begin to show garbled information or lock-up if the DC voltage supplied to it
falls too low, even momentarily. To alleviate this, a brown-out watchdog circuit monitors the level of the power
supply and in the event that the voltage level falls below a certain level resets the display by turning it off, then
back on.
I
2
C LINK TO THE RELAY PCA
While the CPU’s I
2
C communication with the relay board is also routed through the keyboard/display interface,
information passed to and from the relay board via this channel is not recognized by, acted upon or affected by
the circuitry of the keyboard.
04521C (DCN5731)