The invention features methods to detect T wave lability in an individual, and methods of using T wave lability to detect or monitor abnormal cardiac activities. The USB interface circuit has a plurality of input buffers therein and data can be output from one of the input buffers in parallel with an input operation to the other input buffer. If all indexed locations are used, the entries stored in the lookup table can be reordered so that the new entry can be inserted in one of the locations identified by the computed indexes. A transformer converts the alternating current from the wiring to direct current electric and supplies the direct current to the security module. Additionally, a user-side electrical contact is attached with the housing to provide AC power directly to a user’s body.