Measurement-computing WaveBook rev.3.0 Manual de usuario descargar pdf (Pagina 146)

8-12 Theory of Operation,

6-24-99

WaveBook User’s Manual

Multi-Channel Trigger

When the small hardware-limited latencies of the digital (TTL) and single-channel (Channel 1 analog)

triggers are not required, the DSP chip may be used to examine the samples from one or more channels and

to decide if they constitute a pre-defined trigger condition.

The DSP can sample up to 72 input channels and

examine each one to determine if it meets

programmed levels for a valid trigger. This multi-

channel triggering is a two-step process:

1. The DSP examines each of its specified input

signals to determine trigger validity.

After all of the channels have been examined, the

DSP logically combines the individual triggers to

generate the actual trigger. The DSP may be

programmed to generate a trigger if any

individual trigger is valid (OR) or if all triggers

are valid (AND). See figure.

Trigger

Detector

Trigger

Detector

Analog

Input

Signals

Invalid Trigger

Valid Trigger

Re-Arm Command

From Control Circuits

No Trigger

AND

(all)

Trigger

(any)

Trigger

Logic

Trigger

Detector

Multi-Channel Trigger Detection

Trigger validity in a multi-channel environment is determined by the logical relationship among three

elements (slope, duration, and initialization) as discussed in the next section.

Multi-Channel Trigger Types

Trigger Type Slope Duration Initialization

Above-level

Rising Instantaneous Level

Below-level

Falling Instantaneous Level

Above-level-with-latch

Rising Latched Level

Below-level-with-latch

Falling Latched Level

Rising-edge

Rising Instantaneous Edge

Falling-edge

Falling Instantaneous Edge

Rising-edge-with-latch

Rising Latched Edge

The first step in multi-channel

triggering is to examine the input

signals. To determine trigger

validity, WaveBook can examine

each input signal in 1 of 8 ways.

Note: Each trigger type is a

combination of three

elements:

slope, duration,

and

initialization

Falling-edge-with-latch

Falling Latched Edge

Slope (above/rising or below/falling) sets whether the trigger is valid when the signal is:

• Above the trigger level (rising)

• Below the trigger level (falling).

Duration (instantaneous or latched) specifies the action to take if the signal level becomes invalid after it

has been valid:

• Instantaneous triggers become invalid as soon as the signal does; they are used to trigger on the

coincidence of signals, when two or more signals are simultaneously valid.

• Latched triggers remain valid until the acquisition is complete; they are used to trigger on the

occurrence of signals

, when two or more signals

have already become valid

The trigger duration only makes a difference in multi-channel "AND" triggering. In multi-channel "OR"

triggering, WaveBook will be triggered as soon as any channel becomes valid; what happens when a

channel becomes invalid does not matter. In contrast, "AND" triggering waits for all of the triggers to be

valid; and so, latching can be important for rapidly changing signals.

Initialization (level or edge) specifies the sequence necessary for a signal to be a valid trigger:

• Level triggers become valid as soon as they reach or exceed the trigger level, even if they are already

past the trigger level when the acquisition is started.

• Edge triggers first wait until the signal level is invalid. Then they wait for the signal to reach the

trigger level before becoming valid. Thus, level triggers look for a signal level, whenever it occurs;

and edge triggers look for a rising or falling transition that reaches the trigger level.

Examination of the input signals compares two specified signal levels: (a) The trigger level determines

when the input channel is a valid trigger, and (b) the

hysteresis

is the amount by which the channel must

differ from the trigger level for the channel to become invalid.

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