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PORT ADDRESS DECODING
I/O port decoding is done by U404 (74HC154) and U308 (74HC138). These IC's provide active low
outputs which are used to strobe input and output ports, and to provide reset strobes.
Hex
Name
Description
B8
CTR6_CS
LSI Counter/Timer Chip select
B0
INT_CLR
Clears timer interrupt flag
A8
Spare to top PCB
A0
FLAG_CLR
Clears OVERRUN and TRIGGERED Flags
98
P11_CS
HC Counter preset register (C&D)
90
P10_CS
HC Counter preset register (A&B)
88
P9_CS
ECL Counter preset register (C&D)
80
P8_CS
ECL Counter preset register (A&B)
78
P7_CS
Output termination control
70
P6_CS
Key pad input strobe
68
P5_CS
Front panel LED indicators
60
P4_CS
Misc status bits input strobe
58
P3_CS
Internal rate generator control
50
P2_CS
Analog MUX select
48
P1_CS
Output polarity and trigger source
40
DISP_CS
Front panel LCD select
38
Spare to bottom PCB
30
DAC_CS
Write strobe to 12bit D/A
28
CTR5_CS
LSI counter Chip select
20
CTR4_CS
LSI counter Chip select
18
CTR3_CS
LSI counter Chip select
10
CTR2_CS
LSI counter Chip select
08
CTR1_CS
LSI counter Chip select
00
IEEE_CS
GPIB controller Chip select
INPUT PORTS
The two eight-bit input ports on the microprocessor's bus, U406 and U407, are 74HC244 Octal buffers
which are used to read the front panel key pad and miscellaneous status bits from the instrument.
Status Bits (U406)
Name
Description
B7
Always One
B6
Always One
B5
TIMER_INT
1KHz Clock Tick
B4
DROPOUT
Low Power Supply
B3
UNLOCK
80MHz PLL Error
B2
BUSY
Timing Cycle in Progress
B1
OVERRUN
Trigger Rate too Fast
B0
TRIGGERED
Triggered at least once
