The CPU manages all of the internal remote functions including: IEEE-488 and RS-232 remote communication, calibration enable, watchdog timer, serial communication with the FPGA on the A6 Digital Synthesis PCA. See Sheet 1 of the A9 Out-Guard CPU PCA schematic.
The main processor (U10) is a Motorola 68306 that runs on code stored in flash memory (See Sheet 2 of the A9 Out-Guard CPU PCA schematic). The processor directs all internal operations in the Calibrator. It receives commands from the IEEE-488 interface. The RS-232 interface is used in the factory for special testing.
There are two serial ports. One serial port is used for communication with a host PC. The other port, AUX, can be used as a programmable serial output port. When the Calibrator is used with Fluke's hand held scope meters, the port may be used to control the scope meter.
The CPU communicates with the A6 Digital Synthesis PCA through serial communication lines through J1, a 20-pin cable to the A3 Motherboard PCA. All communication with the A6 Digital Synthesis PCA is through differential transceivers and transformers. The differential drivers reduce noise coupling between the guarded and non-guarded CPU circuits. The transformers isolate the guarded and unguarded circuits. The transformers are found on the A6 Digital Synthesis PCA. See Sheet 7 of the A6 Digital Synthesis PCA schematic.
There are several miscellaneous items to note on the A9 Out-Guard CPU PCA. First, there is a watchdog timer circuit, U1. If the CPU loses communication with either the front panel or the A6 Digital Synthesis PCA, the watchdog timer is not signaled which allows a hard reset to the instrument. Secondly, there is the calibration enable switch. When a 57LFC is calibrated, it is recommended that the switch be set to inactivate calibrating the instrument until the normal calibration interval has expired. Finally, a JTAG port is available to allow programming the flash memory through the CPU when changes are made to the software. This can only be used by the factory.
There is over 640 K bytes of static RAM on the CPU board. The RAM has a bank of 128 k x 16 and a bank of 512 k x 16. The is also 512 k x 16 flash memory in U5 and U6. At power-up, the flash is moved into RAM. Program execution is then out of RAM. The real time clock provides the calendar functions for the instrument. If the real-time lock battery wears out, when power off, the instrument will lose its ability to keep track of the calibration cycle. See Sheet 2 of the A9 Out-Guard CPU PCA schematic.
The TMS9914 IEEE-488 IC is used for all remote IEEE-488 communication. There are two buffers that provide the correct driver capability for the IEEE-488 bus. In general, the product complies will all parts of the IEEE-488 standard. Note that U12 and U19 are not installed. See Sheet 3 of the A9 Out-Guard CPU PCA schematic.
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