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Model 8902A
Operation
TALK and LISTEN Status Descriptions
Talk Only Mode If the internal address switches are set to a valid address and the TON (Talk Only)
switch is set to "i", the Measuring Receiver is placed in the Talk Only mode. This mode causes
the instrument to output data each time a measurement is made. Each time the measurement is
completed, data will be output to the bus unless the listening device is not ready for data. If the
listener is not ready, and the Measuring Receiver is in Free Run mode (TO), another measurement
cycle is executed.
Talk Status Only Mode If the five internal address switches and the TON (Talk Only) switch are
set
to "l", the Measuring Receiver is placed in the Talk Status Only mode. In this mode the instrument
is configured
to
send a one-byte data message whenever the bus is in the data mode. The byte sent
is an exact copy of the Status Byte. Each time this byte is successfully sent on the bus, the internal
Status Byte is cleared. The Data Valid (DAV) handshake line is pulsed each time the one-byte Data
message is sent.
Listen
Only
Mode
Listen Only mode is provided to allow the Measuring Receiver to accept
programming from devices other than controllers (for example, card readers).
If the internal LON (Listen Only) switch is set to "1" and the TON (Talk Only) switch is set to
" 0 ,
the Measuring Receiver is placed in the Listen Only mode when the remote enable bus control line
(REN) is set true. The instrument then responds to all Data messages, and the Trigger, Clear, and
Local Lockout messages. However, it is inhibited from responding to the Local o r Abort messages
and from responding to a serial poll with the Status Byte message.
Code Execution in the Measuring Receiver
Some things to consider when programming the HP 8902A are listed below. The following discussion
of the Software Supervisor is included to elaborate on these considerations.
Every two-letter command is acted upon, in order, in software state.
Hardware State equals the Software State only after the end of the
HP-IB
code string AND a
pause occurs allowing the instrument
to
implement the commands it has received (e.g., time
delay between code strings when data is read from the instrument).
Only one measurement mode may be loaded from Software state
to
Hardware state at a time;
e.g.,
D2 PI HI L2. For fail-safe operation, only use one measurement code per code string
before reading a measurement back from the instrument, Refer to Table 3-13, "Measurement
Modes and Codes" for a complete list of all the functions defined as Measurement Modes.
Keyboard or HP-IB commands may not be implemented in hardware stat.e during a measure-
ment if the instrument decides the command is not significant to a particular measurement. Re-
fer to Table 3-13, "Measurement Modes and Codes" and Table 3-
14,
"Measurement Modifiers"
for more information.
The Software Supervisor is the program that controlls overall instrument operation and controls
the implementation of commands received from the external controller. (See Figure 3-5, 'Software
Supervisor Flowchart".) It is a loop that is continuously executed, with measurements made near
the end after checks for proper frequency tuning, proper
RF
and
IF
level, and correct audio range.
After arithmetic manipulation (e.g., for the RATIO function), the program loops back up to display
the measurement.
The Software Supervisor controls the setup of the instrument for a measurement by making use of
two concepts called Software State and Hardware State. Each of these is updated independently.
The Software State is located in RAM and contains a software image of the state of the instrument.
On power-up, the initialization procedure loads the initial Software State from ROM. (Refer to
3-35
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