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PROGRAMMING WITH COMMANDS
Comparing SNAP High-Density and Standard Digital Modules
SNAP high-density digital modules are different in several other ways from standard SNAP
digital input and output modules:
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Standard digital modules contain four points per module; high-density digital modules
contain 32 points per module.
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The points on a standard digital input module are isolated from each other. Points on
high-density input modules are in four groups of eight points; groups are isolated from each
other, but points within a group are not isolated.
•
Standard digital modules have LEDs for each point visible on the module’s top;
high-density modules do not have LEDs.
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Standard digital modules can be placed only in digital slots on the mounting rack;
high-density digital modules can be placed anywhere, even in slots marked “Analog Only.”
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The turn-on/turn-off time is faster for standard digital modules than for high-density
modules. The update time (time for data to pass from the module to the brain) is also faster
for standard digital modules and is determined by the speed of the module itself. In
high-density modules, update time depends on the brain’s analog scanner and is affected
by the number of modules on the rack and how busy the brain is with Ethernet
communication. (For more specific information on turn-on/turn-off and update times, see
the data sheets for standard and high-density modules.) You may find that inserting Delay
commands in your strategy provides more accurate results, especially with counters.
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IMPORTANT: Each point on a standard digital module is given a name when configured in
ioControl or ioManager, and your strategy refers to the point by its name. Points on
high-density modules do not have individual names. HDD modules do not require
configuration, so their points do not appear in the Configure I/O Points dialog box nor on
the Strategy Tree. Because HDD points do not have names, most ioControl commands use
bitmasks to read or write to them.
•
Points on HDD modules cannot be disabled in Debug mode for simulating inputs and
outputs.
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Counting is done differently. See “Counting on High-Density Digital Modules” below for
details.
Counting on High-Density Digital Modules
On standard SNAP digital input modules, any point can be configured as a counter because
counting is done on the brain. We refer to it as “high-speed” counting because it can be very fast,
depending on the speed of the module.
On high-density SNAP digital modules, however, the module itself does the counting, so no
configuration is necessary. The module uses a 16-bit counter, but the brain used with the module
accumulates counts to 32 bits by periodically getting and clearing the module’s counts and
adding each new count to what it already has for each point. Update time varies based on the
number of modules on the rack and Ethernet communication demands on the brain. When using
ioControl User’s Guide
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