5-6
Line power circuits
AC power is applied to the AC receptacle J1001 through the
fuse F101, line switch S101, and line voltage selection
switch S103 to the power transformer T1. The power trans-
former has a total of 10 secondary windings for the various
DC supplies in the instrument.
DC circuits
Numerous DC supplies generate power for the various cir-
cuits within the instrument. Each supply uses a bridge recti-
fier and capacitive filter arrangement, and many supplies are
regulated. Table 5-1 summarizes rectifier, filter, and regula-
tor circuits for the various DC supplies.
5.5.3 Digital board
The various sections of the digital board are discussed below.
Figure 2-4 shows a block diagram of the digital board.
Microprocessor
U631 is a 68302 microprocessor that oversees all operating
aspects of the instrument. The MPU has a 16-bit data bus and
a 21-bit address bus, as well as parallel and serial ports for
controlling various circuits. For example, the RXD1 and
TXD1 lines are used for the RS-232 interface.
The MPU clock frequency of 16MHz is controlled by crystal
Y602. MPU power-on reset is performed by U639, which
holds the MPU RESET line low briefly on power-up.
Memory circuits
ROMs U637 and U638 store the code for instrument opera-
tion. U637 stores the D0-D7 bits of each word, and U638
stores the D8-D15 bits. Note that the digital board includes
provisions for selecting between flash memory and conven-
tional EPROMs; memory type selection is performed by
jumpers W607-W610.
RAMs U635 and U636 provide temporary operating storage.
U635 stores the D0-D7 bits of each data word, and U636
stores the D8-D15 bits.
Semi-permanent storage facilities include NVRAM U634
and battery backed-up RAM U640. These two ICs store such
information as instrument setup and calibration constants.
Note that data transmission to and from these devices is done
in serial fashion. Also, U640 generates the 32.768kHz clock
required to time serial data transmission with the aid of crys-
tal Y604.
A/D converter interface
A/D converter control and data transmission is performed se-
rially through buffer IC U614. Note that data transmission
and reception is controlled by various MPU serial and paral-
lel port lines. A/D converter communication lines include: A/
D_TRIG, used to trigger the A/D; A/D_STB, used to strobe
A/D control data, and A/D_DATA, which receives A/D con-
verter counts from the A/D converter.
RS-232 interface
Serial data transmitting and receiving is performed by the
TXD1 and RXD1 lines of the MPU itself.
U641 provides the necessary voltage level conversion for the
RS-232 interface port.
IEEE-488 interface
U621-U623 make up the IEEE-488 interface. U622, a
9914A GPIA, takes care of routine bus overhead such as
handshaking, while U621 and U623 provide the necessary
buffering and drive capabilities.
Trigger and digital I/O circuits
U612 provides buffering for the digital I/O lines, while U618
provides similar functions for the trigger link, external trig-
ger, and voltmeter complete trigger lines. In addition to buff-
ering, U618 also has additional logic that minimizes MPU
overhead necessary to control the various trigger lines.
Table 5-1
Power supply circuits
Supply
Rectifier
Filter
Regulator
+5VK
+12VK
-12VK
+120VK
-120VK
+1200VK
-1200VK
+5VB
-5VB
+25VS
-25VS
+5VS
+220VS
-220VS
+15VS
-15VS
CR306
CR307
CR307
CR301
CR301
CR304
CR305
CR403
CR403
CR402
CR402
CR201
CR401
CR401
CR202
CR202
C305, C308
C307, C309
C306, C310
C302
C301
C311-C313
C315-C317
C407, C408
C422
C404, C461
C402, C460
C201-C203
C401
C403
C211, C217
C209, C218
Q305
U301
U302
N/A
N/A
N/A
N/A
U408
Q401
N/A
N/A
U207
N/A
N/A
U206
U205
