Earth-Referenced Logic
Unless otherwise noted, components in this discussion are located on
the A1 circuit assembly (34970-66501). The schematics are included
in Chapter 8 starting on page 224.
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.
Chapter 5 Theory of Operation
Earth-Referenced Logic
124
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