Resistance and Capacitance measurements.
Troubleshooting a network segment starts with checking connectivity, measuring the cable impedance making sure it is not open or shorted. Cable impedance is dependant on network type, for example, Foundation Fieldbus uses a 120 Ohm cable terminated at either end of the segment with 120 Ohms. Also, check capacitance, making sure it is within specification. A good cable is essential for proper electrical signal performance.
Network signal electrical performance.
Checking electrical signal performance requires measuring key parameters of the network signal. The 125 includes a Bus Health test mode that is designed to trigger on and capture network signal waveforms. Select a bus type, the ScopeMeter Bus Health mode then runs algorithms to measure key signal parameters like bias voltages, signal low, high and peak-to-peak levels. The bus health mode also measures distortion or signal jitter determining overall signal quality.
Verify signaling meets specification.
The signal parameter measurements are compared to the industry specification, a low and high limit defined by the standard is used to determine if these key parameters are within specification. Display icons are used to clearly indicate which parameters are outside of the industry specification.
Fluctuating or incorrect bias voltages is indicative of grounding or termination issues, while low signal level measurements could mean there is excessive attenuation caused by too long cable runs, too many devices on one segment or simply incorrect network termination. Signal distortion like overshoot is a sign of reflections typically caused by cable imperfections or failing connectors.
Inspecting the signaling for disturbances is simplified with the "eye pattern" display mode. The clearer the center "eye" of the signal display the less distortion and disturbances on the signal. Phase noise and jitter are typical digital signal distortion caused by device receiver transmitter circuits. External electrical interference from high-energy sources such as lighting systems and heavy-duty machinery can be detected by inspecting the eye pattern.
Electrical interference like noise, static discharge and transients can cause network devices to reset, monitoring a segment can capture these events thereby helping to isolate their source.
Once the cabling and signaling parameters are eliminated, network protocol diagnostic tools are used to verify digital protocol performance.
Eliminating cabling and signal quality issues are two critical steps to isolating network performance issues. The Fluke 125 ScopeMeter includes measurement functions and algorithms to verify the integrity of network digital signaling.