Keysight Technologies E4417A Service Manual page 83

4 Theory of Operation
The TTL outputs are connected to the A6 processor assembly via in line resistors
located on the A4 interface assembly. A TTL logic level is output from the
processor assembly when the user defined windows based limits are exceeded.
The TTL inputs are connected to the processor card via in line resistors located on
the interface card and can be used to initiate zero and calibration cycles on the
power meter.
The microcontroller circuits that control all the above functions, and provide
platform for the system software to run on, comprise the microcontroller itself,
memory, and clock and logic circuits. The logic circuits have the function of
ensuring the correct sequencing and decoding of the control signals for the
various peripherals. The crystal oscillator clock circuit is buffered and distributed
to the A4 interface assembly and the A5 daughter assembly. The program
memory for the microcontroller is FLASH EEPROM to allow for in-circuit
programming. The static RAM is split into volatile and non-volatile blocks, with
the power for the non-volatile RAM being connected to the permanent (battery
backed) power supply.
It is advisable to replace the RAM battery A2BT1 every three years. If the battery
NOTE
fails or drops below 3 V the RAM will lose the following information when power
is disconnected from the meter:
– Unit Serial Number.
– Revision letter of Processor Card.
– Unit Option.
– User defined setups.
– User defined sensor calibration tables.
90 Keysight E4416A/E4417A Power Meters Service Guide