IEEE-488 GPIB Interface
16
S
erial
P
olling
E
nable/
D
isable
(SPE/SPD)
Function
Enables or disables the serial polling sequence.
Remarks
The SPE command puts all devices in serial poll mode waiting to be addressed.
The SPD command clears the SRQ bit (bit 6) and ends the polling sequence.
When addressed a device sends its status byte to the controller. A value of 1 for a bit
means that the device condition that bit refers to is true. A value of 0 means that the
condition is false.
The 4421 does not use all bits of the status byte.
Table
7
lists the bits used, along with
a description and how to reset them.
Table 7
IEEE Status Byte Description
CAUTION
During remote operation, periodically monitor the bus service request line. Failure to
detect a service request could result in equipment damage.
Bit
Name
Condition
6
SRQ
Set if a service request is generated by the 4421. If an SRQ
has been received by the controller and this bit is cleared,
other instruments on the bus should be checked to
determine where the SRQ occurred.
Cleared by a serial poll of the 4421.
3
Measurement
Complete
Set when the power meter has completed a reading.
Cleared by requesting a reading over the bus.
2
Reading Underflow
Set when the RF power is underrange and a reading has
been completed.
Cleared by requesting a reading over the bus.
1
Reading Overflow
Set when the RF power is overrange and a reading has been
completed.
Cleared by requesting a reading over the bus.
0
Error
Set if an illegal device-dependent command (IDDC) or illegal
device-dependent command option (IDDCO) was received,
or if the power meter fails the self test.
Cleared by reading the U1 status word. The U1 word
contains details on the error, see
"Status
(Ux)" on
page 21
.
