When using the Serial Digital Interface it is important not to operate a transmission path near to the crash point. Any perturbation to the transmission path could result in the onset of error, or even total failure. A margin (head room) of at least 5 dB at ½ clock frequency (equivalent to fifty metres of Belden 8281 cable for 270 MBit/s CCIR 601 component) is considered normal for safe operation. A simple test is to check head room by adding a fixed length of coaxial cable and confirming signal integrity.

Fixed length cable simulators can be used in place of cable and have many advantages. The Plus Fifty Cable Clone is supplied in a 28 pin DIP package and is designed for pcb mounting in equipment using the Serial Digital Interface. The cable simulator is placed in the main signal path, typically near the output. Energising the integral relay causes a 50 metre length to be added to the signal path to check head room. When no power is supplied to the relay coil the Plus Fifty Cable Clone is effectively zero length. The relay requires a five volt driver and nominally draws 80 mA when energised. The relay coil is floating, allowing either +5v or –5v rails to be used.