HART Communication Module (ROC800-Series)
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The module has four HART input/output channels. Switches on the module board allow each channel to be set up as an input or output channel. A channel that is set as an input can be configured for use in point-to-point or multi-drop mode. A channel that is set as an output can be configured for use in point-to-point mode only. Each channel has Analog Input capability that is intended for diagnostic and backup use, not for primary process variable measurement.
HART superimposes Frequency Shift Keying (FSK) signals on the analog signal. The use of this FSK technique allows digital information to be passed to and from the transmitter on the 4 to 20 milliAmp analog signal. In point-to-point mode, the milliAmp signal is still representative of the primary variable. This mode allows communications with one HART device per analog channel.
In multi-drop mode, as many as five HART devices can be connected (in parallel) to each analog input channel. Like the point-to-point mode, digital communications are superimposed on the 4 to 20 milliAmp signal; however, the analog signal is used only to power the end devices and does not represent any process variable value. With all four analog inputs in the multi-drop mode, the ROC can support a maximum of twenty HART devices.
Performance and speed are greatly improved by a separate FSK modem for each channel. The FSK transmission is independant of other channels. The scan time for one channel does not affect the scan time of any other channel.