HP 3457A Service Manual page 197

It should be noted the auto calibration range is limited; therefore, variable capacitor C332 was included
in the attenuator to serve as a course center adjustment. Normally this will not require adjustment af-
ter leaving the factory: however, a calibration procedure is given in this manual.
8-63. 44491A General Purpose Relay Card
Refer to the 44491A schematic for the following discuss!
n,
The General Purpose Relay Card can be used for four functions; 1) to multiplex signals to the high and
low inputs of the multimeter, 2) to multiplex signals to the amps input of the multimeter (to measure
current), 3) to multiplex signals to an external device through the cards common terminals, or 4) to act
as an actuator,
For the purpose of explanation the card's circuitry will be divided into three functional areas; 1) con-
trol circuitry, 2) drive circuitry, and 3) contact circuitry.
8-64. Control Circuitry
Controlling the position of each relay are the outputs of shift registers U801 and U802. These shift
registers convert a 16 bit serial data string sent from the instrument's inguard controller, A2U501, into a
16 bit parallel output. The parallel outputs are connected to drive transistors U803 and U804 and gates
U806 and U807.
The gates U806 and U807 are used to control the output of inverters. U805a-U805g. These gates are
used to prevent the outputs of both an active high transistor and an active low inverter, which are at-
tached to the same point, from being turned on at the same time.
To identify the card as option 44491A pin 6 of connector P80] is tied to ground. This signal line is
used by the inguard processor as an ID line.
8-65. Drive Circuitry
A matrix scheme of row and column drivers is used to control the configuration of the card's relays.
The drivers are broken up in the following manner.
Two Rows:
Row I- U805a and U803d (K800, 1, 2, 3 and 9)
Row 2- U805b and U803c (K805, 6, 7, 8 and 4)
Five Columns:
Column 1- U80Se and U803b (K800 and K805)
Column 2- U805d and U804a (K801 and K806)
Column 3- U805e and U804d (K802 and K808)
Column 4- U805f and U804e (K803 and K807)
Column 5- U805g and U804b (K804 and K809)
The output of the row drivers are either driven high (+5V) by a transistor being turned on, driven low
(ground) by a low output of an inverter, or left floating which occurs when transistors are turned of f
or when the inverter has a high output. Column drivers have both transistors and inverters tied to the
same point causing the outputs to always be driven high or low.
To open or close a relay requires that one column driver work in conjunction with a row driver. For
instance to set relay K800 requires that column | be driven high while row | is driven low. To reset
HP 3457A Multimeter
8-32