5 Theory of Operation
132 Keysight 34970A/34972A Service Guide
Earth-Referenced Logic
Unless otherwise noted, components in this discussion are located on the A1
circuit assembly (34970-66501).
The earth-referenced logic circuits provide all rear panel input/output capability.
Microprocessor U305 handles GPIB (IEEE-488) control through bus interface chip
U309 and bus receiver/driver chips U310 and U311.
The RS-232 interface is controlled by U305 through U307. RS-232 transceiver
chip U308 provides the required level shifting to approximate ± 9 volt logic levels
through on-chip charge-pump power supplies using capacitors C317 through
C320. Communication between the earth referenced logic interface circuits and
the floating measurement logic is accomplished through an optically-isolated
bi-directional serial interface. Isolator U214 couples data from U305 to
microprocessor U205. Isolator U303 couples data from U305 to microprocessor
U205.
U305 also:
– Controls power to the backplane as well as all rear panel interfaces (GPIB,
RS-232, Alarms, External Triggers). Backplane power is turned on or off based
upon commands received from U205.
– Drives the backplane reset signal (BPRST) based upon commands from U205.
– Monitors the backplane service request (BPSRQ_N) and reports to U205 when
it is asserted.
U306B, U306C, U306D, and U306E drive the alarm outputs. The alarm is a low
true signal at the sub miniature D connector on the rear panel. U306F drives the
channel closed output signal.
The external trigger input is buffered by U304C and U304D.
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