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