ICP DAS USA I-7521, User Manual

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I-7521/I-7522/I-7522A/I-7523/I-7524/I-7527 User’s Manual,Ver2.5 Aug.2015
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The host computer can only send one command string at a time. If
there are two modules, the host computer must send and receive the
commands from module1 before sending/receiving any commands
from module2, which means that there is a time delay between the
two commands. The “synchronize sampling” command is designed
for all input modules. When receiving t
he “#**[0x0D]” synchronized
sampling command, all input modules in an RS-485 network will
perform input functions at the same time and store these values into
the module’s memory. The host computer can then send an “$AA4”
read synchronized data command to read the data separately
4.3
Dual WatchDog Operation Principles
All I-752N modules are equipped with a hardware module watchdog
and a software host watchdog. The I-752N modules are designed for
industrial application, therefore they are able to operate in harsh
environments. As there are usually a great amount of noise or energy
transients in such an environment, the modules may automatically
shut down if the noise becomes too large. The built-in hardware
module watchdog will reset the module if it shuts down or
malfunctions because the noise signal has become too large.
Sometimes, even the Host PC may shut down or malfunction
because of hardware or software reasons. The software host
watchdog can monitor the status of Host PC. If the Host PC fails or is
off-line for any reason, the output state of the I-752N modules will
revert to its predefined safe states for safety protection purposes.
If the RS-485 network is open, any commands from the Host PC will
not be able to be sent to the remote modules. This is very dangerous
in real world applications. The I-752N series output module will force
the output to revert to its predefined safe state for safety
considerations if the host watchdog is enabled. This dual watchdog
feature will greatly increase the system reliability.
4.4
Host WatchDog Application Notes
The software host watchdog is designed to monitor the host
computer. If the host computer fails, the output of the I-752N
modules will automatically revert to its safe state to avoid