Serial Polling (Spe, Spd); Device-Dependent Command Programming - Keithley 1973 Instruction Manual

Table 4-2. Default Values (Status Upon Power Up or After SDC or DCL)
Mode Value Status
Function
—
Reflects front panel selection.
Range
—
Reflects front panel selection.
Relative ZO
EOl KO Send EOl.
Trigger
TO Continuous on Talk.
SRQ MO Disabled
Data Format
GO
Send prefix.
dB DO Off
Terminator YICR LF)
OR LF
Data Logger
BO Disabled
This time, the instrument returns to the default conditions
listed in Table 4-2.
4.2.6 Serial Polling (SPE, SPD)
The serial polling sequence is used to obtain the Model
197/1973 status byte. Usually, the serial polling sequence is
used to determine which of several devices has requested ser
vice over the SRQ line. However, the serial polling sequence
may be used at any time to obtain the status byte from the
Model 197/1973. For more information on status byte for
mat, refer to paragraph 4.3.8.
The serial polling sequence is conducted as follows:
1. The controller sets the ATN line true.
2. The SPE (Serial Poll Enable) command is placed on the bus
by the controller.
3. The Model 197/1973 is addressed to talk.
4. The controller sets ATN false.
5. The instrument then places its status byte on the bus to be
read by the controller.
6. The controller sets the ATN line true and places the SPD
(Serial Poll Disable) on the bus to end the serial polling se
quence.
Steps 3 through 5 may be repeated for other instruments on
the bus by using the correct talk address for each instrument.
ATN must be true when the talk address is transmitted and
false when the status byte is read.
Programming Example—The HP-85 SPOLL statement
automatically performs the serial polling sequence. To
demonstrate serial polling, momentarily power down the in
strument and power it up again. Enter the following state
ment into the HP-85 computer:
REMOTE 720 (END LINE)
S = SPOLL (720) (END LINE)
DISP S (END LINE)
When the END LINE key is pressed after the S = SPOLL state
ment, the computer performs the serial polling sequence.
When the END LINE key is pressed after DISP S statement,
the status byte value (0) is displayed on the CRT. The status
byte has a value of 0 with this example because no bits in the
byte are set. Paragraph 4.3.8 covers the status byte format in
detail.
4.3 DEVICE-DEPENDENT COMMAND
PROGRAMMING
IEEE device-dependent commands are sent to the Model
197/1973 to control its various operating modes. Each com
mand is made up of an ASCII alpha character followed by
one or more numbers designating specific parameters. For ex
ample, dB is programmed by sending an ASCII "D" followed
by a zero or one for turning it off or on. The IEEE bus treats
device-dependent commands as data in that ATN is high
when the commands are transmitted.
A number of commands may be grouped together in one
string. A command string is terminated by an ASCII "X"
character which tells the instrument to execute the command
string.
If an illegal command or command parameter is present
within a command string the instrument will:
1. Ignore the entire command string.
2. Set appropriate error bits in the status byte.
3. Generate an SRQ if programmed to do so.
4-3