Fieldbus Termination


#1

Every field bus segment needs exactly two terminators to operate properly. The terminators are designed to be the equivalent of a 1 µF capacitor and a 100 Ω resistor in series. The terminators serve several purposes including shunting the Fieldbus current(device communication) and protecting against electrical reflections. The primary function of terminators is to act as a current shunt for the control network. Fieldbus communication works by the field device modulating its current draw.

When the devices need to transmit data, the FF devices will act as a current sink. The devices will draw less current to represent a high signal (1, one) and draw more current to represent a low signal (0, zero). As the modulating current of the FF devices is between 15 mA and 20 mA peak to peak, while the modulating voltage in the bus caused by this modulating current is between 0.75 V and 1 V peak to peak. This 0.75 and 1 V is simply a result of the 15 mA x 50 Ω which equal to 0.75 V and 20 mA x 50 Ω which is equal to 1 V.

The 50 Ω resistor is an equivalent of two parallel resistors from the FF terminator. Because of all devices use the same cable, only one device can transmit a message at any given time. Without the proper number of terminators, the signal level will be out of specification and can disrupt the network. Because of all devices use the same cable, only one device can transmit a message at any given time.

Another function of the terminator is to reduce the impact of electrical reflections.