Why Does My RLY-01 SCC Module Default to the Normally Open (NO) State? Hardware: Signal Conditioning Other
Problem: Why Does My RLY-01 SCC Module Default to the Normally Open (NO) State? Solution: The RLY-01 module is driven by the DIO lines on Multifunction DAQ boards. The state of the RLY-01 module is not guaranteed until the board's DIO lines are in output mode. Once in output mode, the default output voltage from the DIO lines is 0V which drives the module to the Normally Closed (NC) state. Since these lines default to the input state which is pulled up to 5V through an internal resistor, you may see some modules default to either state. The only way to guarantee the state of the modules when the DIO lines are in input mode is to place a pull-down resistor as a voltage divider between the DIO lines and ground. Change the DIO line's power-up state to pulled-low by placing a pull-down resistor (RL) between the line and ground so the maximum value is 0.4 VDC. The DIO lines provide a maximum of 2.5 mA at 3.7 V in the high state. Using the largest possible resistor ensures that you do not use more current than necessary to perform the pull-down task. Ensure the value of the resistor is not so large that leakage current from the DIO line, along with the current from the internal pull-up resistor, drives the voltage across the pull-down resistor above a TTL-low level of 0.4 VDC.
Note: The 100kΩ value for the pull-up resistor is an example for the 6025E board. To see what value to use for your board check the E-series user manual at www.ni.com/manuals Use the following formula to calculate the largest possible load to maintain a logic low level of 0.4 V and supply the maximum driving current: V = I × RL → RL = V/I where: V = 0.4 V Voltage across RL I = 46 μA (4.6 V across the 100 kΩ pull-up resistor) + 10 μA (10 μA maximum leakage current) Therefore: RL = 7.1 kΩ (0.4 V/56 μA) This resistor value, 7.1 kΩ, provides a maximum of 0.4 V on the DIO line at power on. You can substitute smaller resistor values to lower the voltage or to provide a margin for Vcc variations and other factors. Related Links: E-series user manual Attachments: 
Report Date: 07/19/2004 Last Updated: 08/02/2004 Document ID: 3BI8CED8 |
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