HP 3336A Operating Manual page 45

Operation
3-24
3-138. Clear. The Clear Message resets instruments to a predefined state. The predefined
state of the -hp- 3336 is its turn on state (see Paragraph 3-10), except that stored instrument
states are retained. The Clear Message can be a universal instruction, resetting all devices on
the bus capable of responding, or an addressed instruction sent to selected devices only.
3-139. Implementation. When the Clear Message is a universal instruction, the entire pro-
gram statement is found in the controller documentation. When it is an addressed instruc-
tion, the syntax and mnemonics of the program statement are found in the controller
documentation. A technical description of the Clear Message implementation is presented in
Figure 3-A-5 of Appendix A.
NOTE
The -hp- 3336 will respond to a universal Clear Message when it is
in the Local operation mode.
3-140. Require Service. The Require Service Message is a request for service which is sent
from a device on the HP-IB to the active controller. Any of the following conditions in the
-hp- 3336 can, when enabled, generate a Require Service Message:
—Received an unrecognizable program string.
—Sweep started.
—Sweep stopped.
—System failure; External reference unlocked or main oscillator failure,
*""OSC FAIL"'
All conditions that cause a Require Service Message from the -hp- 3336 are disabled (mask-
ed) at turn-on. The condition or conditions that will cause a Require Service Message are
enabled using the Data Message, Paragraph 3-151. The Require Service Message is com-
pletely independent of all other bus activity. It is sent on a single line (wire) called the SRQ
Line, whose state is either true or false. This line is shared by all devices on the HP-IB. If the
controller is programmed to respond when a Require Service Message is received, the con-
troller must determine which device or devices are requesting service. This is accomplished
by conducting a Serial Poll. Each polled device responds by sending a Status Poll. Each poll-
ed device responds by sending a Status Byte which indicates, among other things, whether or
not the device requested service. Serial Polling and Status Byte Messages are fully explained
in the discussion of the Status Byte Message (see Paragraph 3-142, Status Byte). The Require
Service Message will be cleared when the device sending it is polled or the condition causing
it disappears. In some applications, the controller is programmed to interrupt its main pro-
gram and respond to a Require Service Message immediately. In other applications, it may
periodically check the status of the Service Request Line and respond when a request is
discovered.
3-141, Implementation. The Require Service Message originates in the devices on the bus.
A technical description of its implementation is presented in Figure 3-A-10 of Appendix A.
3-142. Status Byte. A Status Byte Message is sent by a device on the bus to the active con-
troller. The individual bits of the Status Byte indicate the status of various instrument func-
tions and whether the instrument requested service. Once the Status Byte of an instrument is in
the controller, the status of the instrument functions may be determined by examining the
truth state of each bit. The controller can be programmed to take the appropriate action bas-
ed upon the functional status of the bus instruments. For example, if bit 6 of the -hp- 3336
Model 3336A/B/C