BNC 6040 - Manual (Page 47)

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THEORY OF OPERATION
Memory and I/O Decoding (Schematic 6040-33, Sheet 2)
Program Memory – ROM:
Z.17, the EPROM, encompasses the full 64K code space. During a program code fetch instruction,
PSEN* will cause the internal output buffer of the EPROM to be placed on the data bus. The internal
output buffer is enabled when OE*, Z17-22, goes low (at the same time Z14 will momentarily point
inward, permitting the data to be transferred into the microprocessor).
RAM and Memory Mapped I/O:
Z25, a decoder, divides the 64K data memory space into eighi 8K seclions. YO selects the RAM (Z24),
Y6 selects the plug-in module, and Y7 selects the 8K allotted for other memory mapped hardware I/O.
Data Memory – RAM:
Read/write data memory or RAM is stored in Z24. a 6264 8K by 8 static RAM. It contains the current
operating parameters and other software variables.
Timer and I/O Decoding (Schematic 6040-33, Sheet 3)
Hardware I/O Data Space:
I/O control, selected by Z25-7 (sheet 2), is realized by mapping all hardware into the memory space
EOOO to FFFF. Selections of specific hardware within this area (such as the 82C54 timers and 82C55
parallel I/O chips) is done by Z35, another decoder.
The first four chip selects (CSO-CS3) go to the 82C54 timers. CS4 selects Z33 (front panel control),
CS5 and CS6 select the ECL timing board interface, and CS7 selects the GPIB interface.
CMOS Counters:
The four 82C54 counter timer chips (Z16, Z20. Z23, Z27) each contain three 16-bit timers that can
generate four independent time delays. The logic connected to the clocks, gales and outputs of these
12 timers are shown on schematic sheets 6, 7, and 10. Sheet 3 shows only the microprocessor
interfacing.
Front Panel Interface (Schematic 6040-33, Sheet 4)
Z33 is an 82C55 Parallel Peripheral Interface device with three programmable 8-bit input/output ports.
Z33 is selected by CS4 from Z13 (on sheet 3).
Keypad Decoding:
Port A outputs the keyboard scan pattern to the two 4-bit wide columns in the keypad matrix. The four
rows on the 8x4 keypad map are input on the lower four bits on port C. Key decoding works as follows.