Keithley 708A, Инструкция по эксплуатации

Страница: 120 / 167

Principles of Operation
6-14
6.7.2 Control signals
Bused control signals are managed by U108 (63B21 PIA).
Bused outputs are driven by U134 (7438 open-collector
NAND) and are buffered for input with U136 receivers.
The SLAVE (low true) signal selects the external trigger
source.
In slave units, the SLAVE signal is asserted. This causes
multiplexer U135 to select the M/S TRIGGER (low true) sig-
nal for the trigger interrupt. Slaves are disabled from driving
the M/S TRIGGER signal by the U134 NAND.
In master or stand-alone units, SLAVE is negated, and mul-
tiplexer U135 selects the External Trigger Input BNC for the
trigger interrupt. This also enables the unit to drive the M/S
TRIGGER signal via the U134 NAND.
The ALLREADY signal is wired -ORed so that it is negated
whenever any unit in the master/slave loop has negated its
IMREADY (low true) signal when receiving and processing
data. When a slave unit is powered down, relay K101 pro-
vides a path to digital ground, simulating a negated
IMREADY signal. This provides positive indication to other
units on the master/slave loop that one of the units is not
running. Power-up software waits until all units have
asserted their IMREADY signal.
Assertion of M/S TRIGGER when the ALLREADY signal is
false causes the slave units to reset to a known state. Assert-
ing M/S TRIGGER with ALLREADY true triggers the
slaves.
6.8 Digital I/O
The digital input and digital output ports are two separate
interfaces contained on separate DB25 connectors. A simpli-
fied schematic of the circuitry is shown in Figure 6-12.
Digital inputs are managed by U132 and U133 (74HCT373
transparent latch), which is decoded as a port on the micro-
processor bus by PAL U116. U133 latches in the states of
lines IN(0:7), and U132 latches in the states of IN(8:15)
when the microprocessor reads the digital input port.
Digital outputs are managed by U118 and U119.
6.9 IEEE-488 bus interface
The Model 708A has an IEEE-488 standard interface that
allows the instrument to be programmed from a system con-
troller. Commands can be sent over the bus to the instrument,
and data can be requested from the instrument.
The IEEE-488 interface is made up of U109, U110, and
U112. U109 is a 9914A GPIA (general purpose interface
adapter), while U110 and U112 are interface bus drivers.
The GPIA simplifies microprocessor interfacing to the
IEEE-488 bus because many control sequences take place
automatically. For example, when the microprocessor writes
to the GPIA data output register, the handshake sequence is
performed automatically. Without the GPIA chip, complex
microprocessor routines would be required.
On the microprocessor side of the GPIA, data transmission
is handled much like any other data bus transaction. Micro-
processor data access is performed through the D0-D7 lines,
while A0-A2 (the three least significant address lines) select
among the 14 internal registers (seven read, seven write) of
the GPIA. Chip selection is performed by the SELECT 9914
line.
The output of the GPIA is in standard IEEE-488 format. All
of these lines are active low with approximately zero volts
representing a logic one:
• Eight data lines (D1-D8).
• Three handshake lines (DAV, NRFD, NDAC).
• Five management lines (ATN, REN, IFC, SRQ, EOI).
The two IEEE-488 bus drivers, U110 and U112, bring the
drive capability of the interface up to the requirements of the
IEEE-488 standard, which includes provisions for up to 15
devices on the bus at once. The outputs of the bus drivers are
connected to J1011, a standard IEEE-488 connector.
6.10 Power supplies
The major component of the power supply is a universal
AC-input switching power supply. This supply contains an
internal in-line fuse and an autoranging input, which auto-
matically adjusts to the voltage of the input mains. The
power switch is located on the display board and is actuated
through a mechanical extension of the POWER button. The
AC input is also wired to the display board.
The switching power supply provides reg6VDC for
the relay power circuitry. These voltages are routed to the
switch card via the digital board and backplane. On the
digital board, the +6V is regulated to +5V using one
LM2940 regulator. The LM2940 su5V to the display,
digital circuitry, backplane, switch cards, and micro-DIN
connections.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com