Keithley 194 Instruction Manual page 165

PRINCIPLES OF OPERATION
In a similar manner, U304, U321, and U341, operate together
to multiplex the data bus. U304 is an octal bus transceiver
with 3-state outputs (74LS645). U321 and U341 are octal
D-type flip-flops with 3.state outputs (74LS374). When the
MPU is accessing memory, the U304 outputs will be on
and the outputs of U321 and U341 will be off. U321 and
U341 also act to convert the lh-bit A/D data word into two
bytes that can be stored in an S-bit wide memory location.
U341 controls the low byte, and U321 switches the high
byte. Thus, only one of the three data bus multiplexing
ICs will be turned on at any given time.
6.56 Digital Control-to-Microprocessor
Interfacing Circuits
The microprocessor accesses numerous circuits in the
digital control section through U315 and U323. U315 is a
PIA (Peripheral Interface Adapter), 6821, while U323 is a
VIA (Versatile Interface Adapter), 6522. Each of these ICs
has two R-bit programmable I/O ports. U3l5 allows the
MPU to read the address counter, and it also performs
other functions such as trigger selection. U323 provides
numerous control functions including run/stop control,
trigger slope control, address counter clear, as well as the
data, clock, and strobe signals for the SO-bit serial control
word.
6.5.9 Non-Volatile Memory Circuits (NVRAM)
Calibration constants and operating parameters stored with
the setup mode are stored in the two NVRAM ICs, U301
and U302 (2443). These ICs also store identifying informa-
tion as to the type of module installed in channel 2 loca-
tion. Each device is organized into 16 words of 16 bits each.
Data transfer to and from these devices is done serially
through the PIAs located on the I/O board.
6.6 MICROCOMPUTER
All Model 194 operation is supervised by the microcom-
puter. The following paragraphs describe the fundamen-
tal operating principles behind this important section of
the unit. First a block diagram is given, followed by a more
detailed description of each part of the microcomputer.
While reading though the various descriptions, it will be
necessary to refer to drawing number 194.106, pages 2 and
3 located at the end of Section 8.
6.6.1 Microcomputer Block Diagram
A simplified block diagram of the microcomputer is shown
in Figure 6-6. The 68008 microprocessor controls all aspects
of instrument operation. Software for the MI'LJ is contain-
ed in ROM ICs. The clock circuits generate a 1OMHz clock
for the processor and A/D converter circuits as well as a
5MHz clock for the IEEE-488 interface circuits. The decoder
circuits segment the memory into 64K blocks. The control
bus carries those signals necessary to interface the MI'U
to memory circuits and peripheral devices. The microcom-
puter has a single RAM IC that is used for working storage.
The data bus is eight bits wide, with the data lines carry-
ing the DO-D7 designation. Although the MPU has 20 ad-
dress lines (AO-A19), only 16 of those (AO-A15) are carried
through to most circuits.
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