A2 Isolator Board Circuits (657Xa Series Only); A1 Front Panel Assembly - Agilent Technologies 6571A Service Manual

The desired digital interface function is selected by placing a jumper in a header (J106) on the A2 GPIB Board. Appendix
D in the Power Supply Operating Manual describes how to select one of these functions and how to make the appropriate
external connections to the DIG CNTL connector on the supply's rear panel. Another jumper position on the header selects
the SA (signature analysis) mode, which is used for troubleshooting (see Chapter 3).
The A2 Board has a bias supply regulator IC (U121) that provides +5V (with respect to earth ground) for the primary
interface circuits and the bias voltage for the front panel board circuits, the LCD, and the keypad. The A2 Board also has a
line or bias voltage detector IC (U101) that generates a power clear signal (PCLR). This signal initializes certain primary
interface and front panel circuits when normal ac line voltage is applied, and also shuts these circuits down when the line
voltage drops below the required minimum.

A2 Isolator Board Circuits (657xA Series Only)

The isolator board performs the following two functions:
1. Creates a +5V bias voltage.
2. Provides isolation between the PCLR, RxD, and TxD front panel signals and similar signals received from the A1
Main Board.
When power is turned on to the power supply, an isolated AC signal from XFMR T1 in the secondary circuits is applied to a
+5V bias supply (U805) on the isolator board. The bias supply produces a +5V BIAS output voltage that is routed to the
front panel circuits.
At the same time, a low SPCLR* level from the secondary circuits is applied to optical isolator circuit U800. It is then
routed as a low PCLR* level to the RESET* input of the front panel microprocessor. This low level keeps the
microprocessor temporarily disabled during power turn on for a short time interval. After a delay of 40 ms, SPCLR* goes
high and the microprocessor is enabled. By inhibiting microprocessor operation for 40 ms, any erroneous operation (due to
a rising but yet unstable +5V) is prevented until the +5V BIAS voltage fully settles.
When power is turned off or is removed, SPCLR* goes low immediately to disable the microprocessor in order to provide a
graceful shutdown of the power supply as the +5V falls to zero volts.
Note
PCLR* is generated in the GPIB Board for Agilent 667xA models. For Agilent 657xA models, it
originates at the main board secondary circuits and is routed to the Isolator Board.
The isolator board includes three separate optical isolator circuits that isolate the front panel signals: RxD, TxD, and PCLR*
signals from the SRx, BSTx and SPCLR* signals at the secondary interface circuits.

A1 Front Panel Assembly

The power supply Al Front Panel Assembly contains a circuit board, keypad, liquid crystal display (LCD), and the power
on/off switch. Circuit details are shown on the Al Front Panel Board schematic.
Front Panel Circuit Board A1 contains microprocessor circuits (microprocessor U3 and ROM U4) that decode and execute
all front panel keypad commands. These are transferred to the power supply output via the serial I/O port to the A2 board
GAL (gated-array logic) IC and isolators, and to the secondary interface circuits on the A5 Control Board. The front panel
microprocessor circuits also process power supply measurement and status data received from the serial I/O port. This data
is displayed on the LCD.
IC EEPROM (electrically erasable programmable read-only memory) (U6) on the A1 Front Panel Board stores data and
configuration information. This information includes calibration constants, GPIB address (667xA Series only), the present
programming language, and model-dependent data such as the minimum and maximum values of voltage and current. One
of the EEPROM storage locations holds a checksum value used to verify the integrity of this EEPROM data. Access to the
78 Principles Of Operation