Calculating Shunt Capacitance in Twisted Pair Sensor Cabling
Twisted pair cabling shunt capacitance equals mutual line capacitance plus half the line-to-shield capacitance, setting hard limits on sensor signal bandwidth.
Standard Iec 61158 2
Physical Layer
Industrial communication specifications define the physical layer and signaling methods for fieldbus networks used in industrial control systems. Under these guidelines, standard iec 61158 2 establishes the transmission speed, electrical characteristics, and power distribution rules for digital communications. It is designed to ensure interoperability between field devices and controllers from different manufacturers.
Signal Encoding
The protocol employs Manchester biphase-L coding to transmit both clock and data signals over a single twisted-pair cable. This encoding method ensures that a transition occurs in the middle of each bit period, which allows the receiver to synchronize its clock with the transmitter. It also maintains a zero dc bias on the signal line, facilitating galvanically isolated connections through transformers.
This self-clocking nature is critical for maintaining robust data rates across long cable lengths in high-noise environments.
Power Distribution
Intrinsically safe systems can distribute power to connected devices over the same signal cable, reducing wiring complexity in hazardous areas. The standard defines specific current and voltage limits to prevent sparks or thermal runaway that could ignite explosive gases. Special barriers and power conditioners must be used to isolate the field network from power supply disturbances.
Cable Specification
Shielded twisted-pair cables with specific impedance, resistance, and attenuation characteristics are required to meet the signaling demands of the standard. Cable Type A, for example, is specified with a characteristic impedance of one hundred ohms and is optimized for long distances. Testing the installed cable before commissioning ensures that attenuation does not exceed the allowed limits.