SNAP High-Density Digital Module User’s Guide
11
11
Chapter 2
2: Usage Notes and
Specifications
This chapter contains the following information:
COMPARING HIGH-DENSITY AND 4-CHANNEL DIGITAL MODULES
SNAP high-density digital modules differ in several ways from 4-channel SNAP digital modules. A few
important differences are discussed below; see the table on the next page for more.
Communication with the Processor
One of the main differences between 4-channel and high-density modules is in how the processor (brain or
on-the-rack controller) communicates with them on the mounting rack. Four-channel SNAP digital modules
communicate with the processor through direct wiring; but SNAP high-density modules communicate as
analog and serial modules do, over an internal bus built into the rack and using the processor’s analog
scanner.
This different communication method means that:
•
Communication with the processor (
update time
) is generally slower.
•
Communication speed is affected by how “busy” the processor is—that is, how many modules it talks to
and how many Ethernet communications the processor is handling at the same time.
Counting
Another difference between 4-channel and high-density digital modules is in counting. For 4-channel SNAP
digital input modules, counting is done on the processor. We refer to it as “high-speed” counting because it
can be up to 20 KHz, depending on the speed of the module.
For high-density SNAP digital modules, however, the module itself does the counting. The module uses a
16-bit counter (which goes up to 65,535), but the processor used with the module accumulates counts to 32
bits (4,294,967,295) by periodically getting and clearing the module’s counts and adding each new count to
Comparing High-Density and 4-Channel Digital Modules
