HP 3457A Service Manual page 182

enabled by input multiplexer U507b, pin 9. Multiplexer U507b selects one of three inputs as the
counter enable input (U506, pin 7). The three inputs are either Slope Control A from U508a pin 1,
Slope Control B from U508d pin 13 or the frequency gate signal from U503b pin 9. This time becomes
the time interval that is measured. Upon completion of a time interval measurement, the data present
in U506 is read by the MPU (US501) and combined with the internal MPU counter (U501, pin 14) to
determine the total time interval measured.
8-37. Frequency Counter
The AC assembly, A3, provides a convenient front end for frequency measurements, The ranging
capability can accept input signals over a wide dynamic range of 10mV - 300 volts and frequency
measurements can be made on the function selected by the FSOURCE command. First, an A/D
measurement is performed to determine the optimum range, voltage or current, for the frequency
measurement, This provides input signal conditioning without user intervention.
The A/D converter has external digital hardware used for run-down slope TIME INTERVAL
measurements. Since both frequency and A/D results can not be performed simultaneously, the
frequency counter utilizes this same hardware to accumulate a portion of its result. Frequency is
defined as CYCLES/SECOND. This can be interpreted as ZERO CROSSINGS/TIME INTERVAL and
is precisely what the multimeter has done.
8-38. Floating Common Microprocessor
The floating common MPU US501 is responsible for all aspects of the Analog Section operation. It con-
trols all the measurement dynamics and adds the calibration correction to the finished reading. For ex-
ample, during the A/D conversion process, MPU 501 is completely occupied with control and interac-
tion with A/D hybrid USI1, input hybrid UI01 and external logic. This maintains a constant conver-
sion time. For each measurement configuration, the Al Main Controller sends the appropriate calibra-
tion constants to US01. These constants are used to correct the measurement just completed.
Analog section set-up is performed through serial registers in essential areas. Communication to these
registers is accomplished
through a common
serial data path, SDO (Serial Data Output) generated
by
US501. Individual registers are addressed to receive data by clocking data only to the desired register
through the SDCn control lines which are also generated by USOI. There are five control lines,
SDC2-SDC6, that address serial registers. The register number, the name of the register and the com-
ponents switched are as follows.
HP 3457A Multimeter
8-17