VTI Instruments Corp.
76
EX2500A Trigger Mechanisms
It is easiest to think of each trigger destination as the output of a 27-to-1 multiplexer, where the
source selected in the pull down menu is the multiplexer input that is routed to the destination.
There are three categories of triggers in this hybrid interface: LXI, VXI, and LAN Event. At
power up, the LXI triggers are disabled (driven mode with MLVDS driver output enables
disabled) and are in the low state, as indicated in the
Status
column. The VXI backplane TTL
triggers are open collector signals and are normally implemented with falling edges per
observation B.6.7 of the VXI specification. The user should keep this in mind when implementing
LXI to VXI TTL trigger routes.
LXI
T
RIGGER
C
ONTROLS
In the
LXI Trigger Controls
section, eight LXI Trigger Bus channels are selectable from the drop
menu which can, when selected as the
Source,
be routed to any other trigger destination.
Referencing Figure 6-1, a user is able to configure each LXI trigger channel independently. When
the “output enable” box is not selected, the LXI Trigger Bus channel is configured as an input.
Selecting the “output enable” box configures the LXI Trigger Bus channel as both an input and an
output. Another user selectable LXI option is channel mode.
The LXI Wired Trigger Bus operates in two modes:
Wired-OR
and
Wired-OR bias
(also known as
“Driven”). Wired-OR mode allows all devices on the bus to manipulate the trigger line. One and
only one instrument on the bus must be set to Wired-OR bias mode. This instrument maintains a
bias in a similar to the way a pull-up resistor provides a bias on a TTL open-collector wired-or
bus. All other instruments on the line must be in Wired-OR mode. On the trigger bus, instruments
that are not driving the bus are said to be in “Disabled” drive mode. The instrument driving the
bus is said to be in “Driven” drive mode, while all other instruments driving the bus are in
“Wired-OR” drive mode. As multiple instruments can attempt to drive the bus simultaneously, any
device attempting to drive the bus “high” or “true” will drive the bus if other devices attempt to
drive the bus “low” or “false”. As a result, events can be initiated by either the first instrument to
drive the bus high or by the last instrument to return the bus to its low state.
The
Manual Control
column allows the user to 1) set an LXI trigger channel low or high (via the
“Low” and “High” buttons) or 2) pulse the channel from high to low (via the “Pulse” button) The
Status
column is a static indication of the current state of the LXI trigger bus channel. Due to
software layer delays, state changes are not indicated when pulsing the trigger bus channel.
VXI
T
RIGGER
C
ONTROLS
The
VXI Trigger Controls
section of the
Trigger Configuration
page represents another group
of trigger sources/destinations available on the EX2500A. These triggers include the eight TTL
backplane triggers, the two ECL backplane triggers, and the front panel external trigger. Although
not destination selectable, the 10 MHz clock source, whether the on-board oscillator or some other
source, can be used as a trigger source.
The eight TTL backplane triggers can operate at a maximum frequency of 12.5 MHz. In some
instances, such as routing a 20 MHz LXI trigger bus source to a VXI TTL trigger destination,
source to destination selections directly attainable. However, by using a short series of source-
destination selections, this can be accomplished. If, for example, the source for TTL0 is set to
LXI0 (and Invert is selected) and the source for LAN0 is set to TTL0, LAN0 events can be
generated by TTL0 their connection with LXI0.
Asserting a trigger listed in the
VXI trigger controls
section drives that trigger to a TTL low. De-
asserting a trigger listed in the
VXI trigger controls
section allows the trigger to be pulled up in
the case of the TTL and ECL triggers are driven high by the EX2500A in the case of the external
trigger. The pulse button here creates a high-low-high transition on the corresponding trigger.
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