Ieee488Bus; Input/Output Circuitry; Memory Mapping - Keithley 595 Instruction Manual

THEORY OF OPERATION
PB6 lie, the PB7 line, and the DS line. Signals generated
by this circuiky include a line which controls the ROM chip
select, a signal line that controls the RAM chip select, and
circuitry which enables the display control and IEEE488
bus circuits. Additional device seletion signals include the
memory paging signals. Two signals divide the 16K ROM
aea into 4K pages, while a third signal divides the 2K RAM
area into two 1K pages.
Because of the paging scheme employed, several devices
can occupy a given address within the miaoprocessor's ad-
dressing space. Table 7-2 gives the general address range
for each device.
Table 7-2. Memory Mapping
Device
ANALOG OUT
RAM uJ107)
ROM (U106)
Display Control (Ull.0)
Display Control (lJ114)
IEEE-488 Bus (Ull3)
Address Range
~-$04L4
$OOOO-$03FF
"%!F
7.10.4 IEEE-488 Bus
The Model
595
has a standard IEEE-488 interface that allows
the instrument to be programmed from a system controller.
Commands can be sent over the bus to the instrument, and
data can be requested from the instrument as well.
The IEEE-488 interface is made up of Ull3, U118 and UllY.
Ull3 is a 9914 GPIA (General Purpose Interface Adapter),
while TJll8 and U119 are 75161 and 75160 interface bus
drivers, respectively.
The 9914 GPIA simplifies microprocessor unit interfacing
to the IEEE-488 bus because many control sequences take
place automatically. For -pie, when the microprocessor
unit writes to the GPIA data output register. the handshake
sequence is performed automatically. Without the GPIA
chip, complex and time consuming microprocessor unit
routines would otherwise be required.
On the microprocessor unit side of the GPIA, data trans-
mission is handled much like any other data bus transac-
tion. Microprocessor unit data access is performed through
the DO-D7 lines, while the RSO-RSZ lies (which are con-
nected to the three least significant address lies) serve to
select among the 14 ~internal registers (seven read, seven
write) of the integrated circuit; Chip selection is performed
by the CS lime.
The output of the 9914 IC is in standard IEEE-488 format;
the eight data lines @IO1 - DI08), the three handshake
lines (DAV, NRFD, NDAC), and the five management lines
(ATN, REN, lFC, SRQ, and EOI) are all active low with ap-
proximately zero volts representing a logic one. The two
IEEE488 bus drivers, Ull8 and UllY, are necessary to bring
the drive capability of the interface up to the requirements
of the IEEE-488 standard, which included provisions for
up to 15 devices to be connected to the bus at one time.
The outputs of the bus drivers are connected to JlOl.2, which
is a standard IEEE-488 connector.
7.10.5 Input/Output Circuitry
Additional microprocessor unit functions include the con-
trol of the Meter Complete and External Trigger Input,
analog-to-digital converter control, and voltage source
control.
Ate the end of its conversion cycle, the Model 595 sends a
pulse out the Meter Complete jack on the rear panel. This
function is performed by the PB2 line of the microprocessor
unit through UlO2A configured as a bufferfiverter. Diodes
CR104 and CR105 and resistor RX32 protect the circuit
output.
U'lO2D, U105B, and associated components process the in-
coming bigger signal. UlO2D buffers and inverts the signal,
while UlO5B latches the trigger pulse. The pulse is then read
by the miao~processor unit through PB6. FBl is used to reset
the trigger latch once the pulse is read. Note that~PB1 is
also used to read the status of the calibration jumper (WlOl)
during the calibration program.
As with the Meter Complete output, protection of the Ex-
ternal Trigger input is necessary to protect the device from
overvoltage inputs. External Trigger protection components
include RlOl, CRlOZ, and CRlO3.
A/D control information is fed out the PB3 and PB5 ter-
minals through UlO2B. A pulse width modulation scheme
is used with 18 and 50psec pulses representing logic 0 and
logic 1 respectively. A 2OOpec pulse is used to strobe data
into the A/D and serial-parallel control circuits. Note that
this information is used to control the A/D converter as well
as to control the input preamplifier (through relays), set
the ranging amplifier gain, and to control the three phases
of the measurement~cycle. Isolation is provided by opto-
isolator Ul21.
7-19