When the BERT is testing a synchronous device, a CCITT 511 pattern is transmitted. The BERT receiver verifies that the returning data is error-free, at which point the LED is lit.When in async mode, the BERT sends QUICK BROWN FOX messages and waits to receive the FOX messages back before illuminating the GOOD DATA LED. Flow control is achieved by using the CTS interface lead if you are testing DCE or the DTR interface lead if you are testing DTE. When the interface lead becomes inactive the BERT stops transmitting data.All power to operate the Pocket BERT is provided by the unit being tested—no external power supplies to carry around! Just drop the Model 55 BERT into your pocket and you are ready to troubleshoot anywhere.