GE GFK-0787B Input Autotest for GMR Inputs, Calculating Voltage Drops on Tristate Inputs

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Input Autotest for GMR Inputs

During GMR configuration, input autotesting can be individually turned on or off for each input in an input group. The GMR software will automatically test the selected inputs for the ability to reach the alarm state. The ability to diagnose short circuits on inputs depends on whether the circuit is set up as a bistate or tristate input, and on whether the block itself is configured for GMR mode (using the Hand-held Monitor).

HAutotesting checks the ability of the input electronics to recognize both the On and the Off state. During each Input Autotest, some inputs are forced to the Off state by de-energizing the power feed output, and some are forced to the On state via the Genius block electronics. See page 5-6 for more detailed information.

HInput autotesting also detects circuit-to-circuit shorts.

HNote: blocking diodes are required to use the Input Autotest feature. These diodes are in addition to a Zener diode that may be added for line monitoring.

+24V

Optional Zener diode for line monitoring

Source

Genius

Block

See page 5-6 for more detailed information about input autotesting. Also see pages 8-3 through 8-9 for Autotest wiring information.

Calculating Voltage Drops on Tristate Inputs

It is important to consider the field wiring runs required for devices configured as tristate inputs. Devices that are powered by 24V DC will have a voltage-reducing component inserted at the field device to provide an input threshold range for three states. The table on the next page shows appropriate ranges. Wiring runs can reduce the voltage at the input block terminal further, to an inoperable level, depending on the length, conductor, and gauge. Isolation diodes placed before the terminal on the input will also drop the voltage.

Most applications do not have limitations created by these factors. However, to ensure that all input state operations are indicated correctly, calculations should include the field signal voltage, the wire resistance times the length and the voltage drop in any barriers or isolation devices, to determine the actual voltage present at the input terminal.

Additional information about input blocks is located in the Genius I/O Discrete and Analog

Blocks User's Manual (GEK-90486-2).

2-6	GeniustModular Redundancy Flexible Triple Modular Redundant (TMR) System	GFK-0787B
	User's Manual ± March 1995