CHAPTER 4: MAINTENANCE AND TROUBLESHOOTING
SNAP PAC R-Series Controller User’s Guide
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6.
In the Operation Commands list, highlight Store configuration to flash. Click Send Command.
7.
In the Operation Commands list, highlight Restart brain from powerup. Click Send Command.
The new TCP parameters are set.
TROUBLESHOOTING I/O MODULES: FREQUENTLY ASKED QUESTIONS
SNAP Digital Troubleshooting
Q: I have a SNAP digital module that has stopped working. What may have caused this?
A: Failure of SNAP digital I/O modules is very rare and normally caused by misapplication, miswiring, or using
the module at the wrong logic voltage. As with Opto 22’s other digital I/O module families, there is no one
way in which SNAP I/O modules fail. Output modules normally fail in a different way than inputs.
Output modules usually fail open, or half-wave for AC, and the failure is typically caused by the application of
too much current or by degradation of the semiconductor switch by overvoltage. Because the module is
separated into two parts by the optical isolator, it is possible to have an output module that behaves properly
on the logic side but is broken on the field side. The LED will in most cases continue to turn on and off as
commanded by the driving logic device, even though the module has failed.
The first step to troubleshooting a failed output module is to check the fuse. If it is blown, simply replacing it
should get the module working again. If the module continues to blow fuses, analyze the load to see if the
module is really suitable for the application.
When input modules fail, they usually do so on the field side, typically from overvoltage or overcurrent
resulting from overvoltage. Unlike output modules, an input module’s LED normally will not function as
expected when the module has failed.
Q: I have an input module that seems to flicker when a voltage is applied to it. What is happening
here?
A: Some Opto 22 SNAP digital input modules contain full-wave rectifiers. In an AC application it is possible to
destroy one-half of the rectifier and still have the module appear to function. The full-wave rectifier turns into
a half-wave rectifier, causing the module to turn on and off at the line frequency when an input is applied. You
may see this as flicker on the channel status LED, or it may be too fast to see.
Digital logic devices, being much faster than the human eye, can cause serious issues with this failure mode.
For example, a digital counter attached to a failed module will count at the input frequency.
This failure occurs most commonly in applications where the input module is placed in parallel with an
inductive load. When the load turns off, the voltage spike (the back-EMF) goes right through the module,
destroying one-half of the bridge.
To significantly reduce the frequency of this type of failure, place a transorb or large R-C snubber in parallel
with both the load and the input module.
Q: I have an output module driving a load. The load turns on but never seems to turn off, unless I
remove power from the module entirely. What might be happening?
A: This issue may occur when using an output module with a high-impedance load, such as a neon lamp or a
small solenoid. Loads like these often have relatively large initial currents but relatively small “hold-in” currents.
The result is that the off-state leakage current through the module is sufficient to keep the load on, once it has
been turned on.
