5 Theory of Operation
166 Keysight 34970A/34972A Service Guide
Digital I/O
Components in this discussion are located on the A1 circuit assembly
(34907-66501).
A simplified diagram of a digital I/O channel is shown below.
Two stages of latches on the outputs and one set of latches on the inputs provide
synchronous 16 bit writes and reads of the digital ports.
For digital output, data is written to the upper and lower bytes (U201 and U202)
separately, then latched into the output latches U203 and U204 simultaneously.
On a digital input data is latched into the input latches U105 and U106
simultaneously.
MOSFETs are used to provide the low level output, and 74HC240’s are used to
provide the high level output.
During an output low, a logic high level is applied to the gate of the MOSFET
causing it to conduct and creating a low resistance path from the data line to
ZGND. In this state the MOSFET is capable of sinking an externally supplied
current of up to 400 mA. The blocking diodes, CR301-CR308 and CR401-CR408
prevent any current from sinking into the 74HC240’s.
During an output high, a logic low level is applied to the gate of the MOSFET
turning it off and presenting a high resistance between the data line and ground.
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