HP 3457A Service Manual page 199

8-67. Operation of Channel 0
An explanation of the operation of channel 0 is given below. Circuitry used to control channels 1-7
operates in an identical manner to the control circuitry for channel 0.
Closing channel 0 requires both relay K800 and K804 be configured in the set position. To configure
K800 in the set position, row | must be driven low and column | driven high for approximately 10 mil-
liseconds. This will occur as the result of Q4, Q6 and Q8 pulsing low. These three outputs cause the
following events to occur:
© Transistor U803b is turned on driving column | high (this transistor will act as a current
source),
© The output of gate U806c is forced low which subsequently causes the output of inverter
U805c to be driven high (open).
The output of gate U806b is forced low which subsequently causes the output of inverter
U805b to be driven high (Row 2).
© Since all other outputs of U801 and U802 are high, all other column drivers (U805d-U805g
and U804a-U804d) and row driver | (U805a) are driven low.
As a result of these logic states, transistor U803b sources current through the set coil of relay K800 to
the associated logic ground created by the low output of inverter U805a. Current flowing through the
relay coil creates an electromagnetic field which causes K800's to latched in a closed (set) position
After K800 has been latched closed, the shift register will output new data to latch K804 in its set posi-
tion. This is done by driving column 5 high and row 2 low as a result of U802 outputs QI, Q5 and Q6
being low.
Since the relays used on this relay card are latching relays, an active circuit is required to change the
position of the contacts (open or close). Resetting K800 requires that row | be driven high while
column 1 is driven low for approximately 10 milliseconds. Circuit operation to latch relay contacts
work similar to what was described earlier for latching the contacts closed.
8-68. Card Configuration
Holes are left in the circuit board to allow for users to easily configure different channels to fit their
application. Components on the schematic that are have a label with the letters UC are not mounted at
the factory. These "User Configurable" components only indicate mounting holes.
Channels 4-7 have positions to mount RC or RL filters on the inputs. To create an RC filter the selec-
ted resistor should be put in place of JM801-JM804 and the selected capacitor in UC809-812.
Channels 8 and 9 have positions to mount contact protection circuitry. A general contact protection
circuit might consist of a jumper in position UC805-UC806, a resistor in position UC807-UC808, a
capacitor in UC803-UC804 and a metal oxide varistor in position UC801-UC802.
To configure the COMmon terminals of the card, jumper wire needs to be placed in position UC813
and UC814. This will connect channels 0-7 to the COM terminals whenever these channels are connec-
ted to the instrument.
HP 3457A Multimeter
8-34