Digital Board - Keithley 6517 Service Manual

Troubleshooting
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.
Table 5-1
Power supply circuits
Supply Rectifier
+5VK CR306
+12VK CR307
-12VK CR307
+120VK CR301
-120VK CR301
+1200VK CR304
-1200VK CR305
+5VB CR403
-5VB CR403
+25VS CR402
-25VS CR402
+5VS CR201
+220VS CR401
-220VS CR401
+15VS CR202
-15VS CR202

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.
5-6
Filter Regulator
C305, C308 Q305
C307, C309 U301
C306, C310 U302
C302 N/A
C301 N/A
C311-C313 N/A
C315-C317 N/A
C407, C408 U408
C422 Q401
C404, C461 N/A
C402, C460 N/A
C201-C203 U207
C401 N/A
C403 N/A
C211, C217 U206
C209, C218 U205
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.