ance is de- other approach is to surround the signal conductors with a conduc- graded. Only too often, the shield is brought down to a 'pig tail' or tive shield which is grounded to the equipment screen. The function drain wire and terminated to one of the connector pins (the EIA/RS- I CM1 Isolation Capacitance imbalance allows transformer common mode signal to develop a differential mode interference current DM = I CM2 Figure 23.33 Unbalance between feedthrough capacitors 23/26 Safety and EMC High pass filter between each contact if Z>0 Common impedance Z Figure 23.34 Common impedance coupling through filter capacitors 232D interface standard even has a pin allocated to this function, pin using a screened backshell, will compromise the high frequency 1) or, worse still, it is not terminated at all (Figure 23.36). shielding of the system. The effect of no shield connection at all is to nullify the shielding Inexpensive subminiature D-type multi-way connectors are now effectiveness at high frequency. This is perhaps to be expected, but available whose construction adheres to these principles, and if they what is less intuitively obvious is that the pigtail connection is are used correctly they can give good screening. It is quite possible almost as bad (Paul, 1980). The difference in effectiveness between to misuse them either in design or assembly and throw away all their a pigtail connection and a full 360° connection is small below advantages. Most other types of multi-way connector, in particular 3MHz, but can approach 40dB at higher frequencies. It is caused by the popular insulation displacement two part units, have no potential the pigtail inductance appearing in series with the screen, and can for making shielded connections and should only be used for inter- show variations greater than 20dB over small changes in frequency. board connections inside screened equipment, although certain manufacturers are now offering a type of screened backshell for shielded ribbon cable. 23.15.2 Screened backshells 23.16 R