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Service Information
4-8
4.4 Special handling of static-sensitive
devices
CMOS and other high-impedance devices are subject to pos-
sible static discharge damage because of the high-impedance
levels involved. When handling such devices, use the precau-
tions listed below.
NOTE
In order to prevent damage, assume that all
parts are static-sensitive.
1. Such devices should be transported and handled only in
containers specially designed to prevent or dissipate
static build-up. Typically, these devices will be received
in anti-static containers made of plastic or foam. Keep
these parts in their original containers until ready for in-
stallation or use.
2. Remove the devices from their protective containers
only at a properly grounded workstation. Also, ground
yourself with an appropriate wrist strap while working
with these devices.
3. Handle the devices only by the body; do not touch the
pins or terminals.
4. Any printed circuit board into which the device is to be
inserted must first be grounded to the bench or table.
5. Use only anti-static type de-soldering tools and
grounded-tip soldering irons.
4.5 Principles of operation
The following paragraphs discuss the basic operating princi-
ples for the Model 6521/22, and can be used as an aid in trou-
bleshooting the card. The schematic drawings of the scanner
cards are located at the end of Section 5.
4.5.1 Block diagram
Figure 4-6 shows a simplified block diagram of the Model
6521/22. Key elements include the relay data control cir-
cuits, the relay drivers and relays, and the power-up reset cir-
cuit. These various elements are discussed in the following
paragraphs.
4.5.2 Relay control
Card relays are controlled by serial data transmitted from the
host electrometer via the relay DATA line. A total of three
bytes are shifted in serial fashion into latches located in the
card relay driver ICs (U102- U104). The serial data is
clocked in by the CLK (clock) line. As data overfl ws one
register, it is fed out the QS line of the register down the
chain.
Once all three bytes have shifted into the card, the STR line
is set high to latch the relay information into the Q outputs of
the relay drivers, and the appropriate relays are energized
(assuming the driver outputs are enabled, as discussed be-
low). Note that a relay driver output goes low to energize the
corresponding relay.
4.5.3 Switching circuits
Signal switching is performed by relays K101 through K120.
Even-numbered relays (K102 through K120) perform the ac-
tual signal switching, while odd-numbered relays (K101
through K119) maintain a common path for current switch-
ing by maintaining a current path to common when a channel
is open.
4.5.4 Power-on/power-off safeguard
A power-on/power-off safeguard circuit, made up of U105
and associated components ensures that relays do not ran-
domly energize on power-up and power-down. This circuit
disables all relays (all relays are open) during power-up and
power-down periods by holding the OE (output enable) lines
high during these periods.
To 6517
Electrometer
Clock
Data
Strobe
Enable
+5V
User Connections
Power-on
Reset
U105
Relays
K101-K120
Relay
Drivers
U102-U104
Figure 4-6
Block diagram