Telecom applications ask for FMLB levels for various reasons:

• 1. ESD (Electrostatic discharge) and EMI (Electro-magnetic interference) require ground potential to be connected first in order to make sure no signal or power connection- is done to the PCB, when inserted during power on. This function is dedicated to full-FMLB where the pins are 1.75 mm longer than standard.
• 2. When the PCB is further plugged in, it is desired to have power connected as next sequence in order to prevent latching effects in IC's (happens if signals are connected before power). For this reason semi-FMLB pins are available which are 0.85 mm longer than standard.
• 3. After 1. and 2. the signal contacts shall be connected next. These contacts have "zero-level".
• 4. In order to "start" the circuitry functions on the PCB, designers want to have a signal on the board, when all contacts are fully inserted. This signal normally is created by a series circuitry of two Last-Make-First-Break pin positions which should be located in the middle row (row C) close to the ends of the connector.

The IEC 102-4-100 specification for metric racks defines a maximum angular misalignment of the boards during insertion of +/- 0.5¡ in longitudinal and transverse axis. The tangent of 0.5¡ equals 0.00872 which - over a height of a 4 SU connector (95 mm) - accumulates already to 0.82905 mm. In order to assure the mating sequence this must be the minimum distance between the various FMLB and LMFB of a 4 SU connector. MOLEX OMNIGRID connectors do comply with these requirements as we divided the IEC suggestion for one extended mating length of +1.75 mm only into two halves of 0.85 mm each and added a lower level LMFB of - 0.85 mm.