IEEE-488 Reference
3-18
3.9
Trigger Model (IEEE-488 operation)
The following information describes the operation process of
the Model 6517A over the IEEE-488 bus. The flowchart in
Figure 3-14, which summarizes operation over the bus, is
called the Trigger model. It is called the trigger model be-
cause operation is controlled by SCPI commands from the
Trigger subsystem (see paragraph 3.23. Notice that key SCPI
commands are included in the trigger model.
Idle and initiate
The instrument is considered to be in the idle state whenever
it is not operating within one of the layers of the trigger mod-
el. The front panel ARM indicator is off when the instrument
is in the idle state. While in the idle state, the instrument can-
not perform any measure or scan functions. Over the bus,
there are two SCPI commands that can be used to take the in-
strument out of the idle state; :INITiate or :INITiate:CON-
Tinuous ON.
Notice that with continuous initiation enabled (:INIT:CONT
ON), the instrument will not remain in the idle state after all
programmed operations are completed. However, the instru-
ment can be returned to the idle state at any time by sending
the *RST command, the *RCL command, or the
SYST:PRES command.
Trigger model layers
As can be seen in Figure 3-14, the trigger model uses three
layers; Arm Layer1, Arm Layer 2 and the Trigger Layer. For
front panel operation, these layers are known as the Arm
Layer, Scan Layer and Measure Layer.
Once the Model 6517A is taken out of the idle state, opera-
tion proceeds through the layers of the trigger model down
to the device action where a measurement occurs.
Control sources —
In general, each layer contains a control
source which holds up operation until the programmed event
occurs. The control sources are summarized as follows:
• IMMediate — With this control source selected, event
detection is immediately satisfied allowing operation to
continue.
• MANual — Event detection is satisfied by pressing the
TRIG key. Note that the Model 6517A must be taken
out of remote before it will respond to the TRIG key.
Press LOCAL or send LOCAL 27 over the bus to take
the instrument out of remote.
• BUS — Event detection is satisfied when a bus trigger
(GET or *TRG) is received by the Model 6517A.
• TIMer — Event detection is immediately satisfied on
the initial pass through the layer. Each subsequent de-
tection is satisfied when the programmed timer interval
(1 to 999999.999 seconds) elapses. A timer resets to its
initial state when operation loops back to a higher layer
(or idle). Note that TIMer is not available in Arm Layer
1.
• EXTernal — Event detection is satisfied when an input
trigger via the EXTERNAL TRIGGER connector is re-
ceived by the Model 6517.
• TLINk — Event detection is satisfied when an input
trigger via the TRIGGER LINK is received by the Mod-
el 6517.
• RTCLock — Event detection is satisfied when the pro-
grammed time and date occurs. Note that the real-time
clock is only available as a control source for Arm Lay-
er 1.
• HOLD — With this selection, event detection is not sat-
isfied by any of the above control source events and op-
eration is held up.
Control source loops —
As can be seen in the flowchart,
each layer has three paths that allow operation to loop around
the control source. These three paths are described as fol-
lows:
• :DIRection (Source Bypass) — When a source bypass
is enabled (:DIRection SOURce) and the EXTernal or
TLINk control source is selected, operation will loop
around the control source on the initial pass through the
layer. If programmed for another event detection in the
layer, the bypass loop will not be in effect even though
it is still enabled. The bypass loop resets (be in effect) if
operation loops back to a higher layer (or idle).
In Arm Layer 1 and Arm Layer 2, enabling a source bypass
also enables the respective output trigger. In the Trigger Lay-
er, its output trigger is always enabled and occurs after every
device action. See Output Triggers for more information.
• :IMMediate — Each time an :IMMediate command is
sent, operation loops around the respective control
source. It is used when you do not wish to wait for the
programmed event to occur (or when the HOLD control
source is selected). Note that in Arm Layer 1 and the
Trigger Layer, :IMMediate also loops operation around
the delays.
• :SIGNal — Same function as an :IMMediate command.
Delays —
Arm Layer 2 and the Trigger Layer have a pro-
grammable Delay (0 to 999999.999 seconds) that is asserted
after an event detection. Note, however, that a Delay can be
bypassed by sending an :IMMediate or :SIGNal command.
