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8.2.2 PROBLEM - CAN SEE SOME BUT NOT ALL DEVICES ON THE
NETWORK
Situations causing this issue include:
1. Devices with the same barn ID number. Run the barn ID utility and correct the problem.
2. Device cannot be seen may be unplugged or may have failed. Check for bad connections on the device or try
plugging in another device.
Note!
When plugging in a new device, do not expect it to appear in the correct barn position immediately. First,
it must be configured and given a barn ID number. Refer to
Section 6.1
for this procedure.
8.2.3 FINDING AN OPEN CIRCUIT ON THE CAN NETWORK
Using a digital ohms meter, test the resistance of the network cabling at the beginning of the network to be certain
there are no shorts and that the network resistance is within acceptable range, between 50 and 70 ohms. To do this, first
make sure all Pulse MD Units are unplugged. Then, place the leads of the test meter on the two wires coming out of the
network initiation module and read the ohms resistance. If the ohms meter reads around 120 ohms, it indicates that the
circuit is open or that the initiation and termination modules are not properly installed. This problem must be found and
eliminated for the network to operate properly.
In addition to ohms readings, troubleshooting for connection problems can be performed by plugging in the devices. To
confirm adequate impedance, plug in one device at a time while simultaneously reading the resistance on the network.
If plugging in a device drastically changes the ohms reading, stop and look for the problem.
An open circuit is one in which one of the wires transmitting network signals has broken.
Figure 34
is an illustration of
a typical network with an open circuit. The area in the circle represents an open circuit caused by the blue wire that has
been cut or broken.
To check for an open circuit, measure the resistance on the network system. Disconnect the wires from the network
controller. Then, using an ohms meter, connect the meter probes to the beginning of the network (near the controller).
If there is no broken wire and the network impedance is proper, the meter should read between 55 and 65 ohms.
This is because each module consists of a 120 ohms resistor installed in parallel on the network (initiation/termination
module). If the meter reads 120 ohms, assume there is a broken wire.
To locate the area where the wire is broken, start by removing the network termination module as shown in
Figure 34.
Take an ohms meter and unplug the first device on the network. Place the probes of the meter into the sockets for the
blue and white wires and read the resistance. If the reading indicates at or near infinite resistance, the broken wire is
between the computer and the first device on the network. Reattach the first device and continue to the second, third
and so forth. When there is a reading of infinite or near infinite resistance, begin looking for the broken wire between
the last reading at or near 120 ohms and infinite.
!
