CABLE PRESENT DETECTOR
Original Publication Date: 2000-Nov-27
Included in the Prior Art Database: 2003-Jun-12
Background Information High performance electrical data links typically require differential drivers and receivers. This is particularly true when the electrical length of the interface is long relative to a bit time. Differential signals improve noise immunity and radiate less energy than single ended signals. Differential signals do have a major drawback however. When no cable is present or the differential receiver is not connected to any source, the differential nature of the input causes spurious noise out of the receiver. This happens because the two differential signals are floating and the receivers typically have large gains that amplify any small difference between the two inputs. Designers of such receivers have modified their designs so that when no signal is present a pull-up or pull-down resistor biases the input to one logic level or the other. This causes the receiver to behave well when no signal is present at the expense of reduced performance under normal operation. This effect can be corrected but further complicates the design. In general, detecting if a cable is present, properly mated, and correctly placed is a problem. The problem outline above serves to illustrate just how complicated the problem can be. An illustration of a typical card connector for a cable is shown in Figure 1 where "S" is a signal pin, "G" is a ground pin, and "R" is a resistor. There are multiple signal pins shown (which could be differential) and multiple ground pins shown. Ground pins are used to terminate signal shields and to ensure a reliable low impedance ground connection between card and cable. A signal pin is isolated from the ground bus at the card connector. This pin is connected to a pull-up resistor and to a normal single-ended receiver circuit. When no cable is plugged into the connector all the ground pins from the cable would be connected to the ground pins on the card connector including the isolated pin with the pull-up. This action will cause the receiver input to go to ground or a logic zero level indicating the cable is present.