Single-ended and differential signal voltage levels are not compatible and must not be mixed on the same SCSI bus.
Single-ended
Single-ended cable configurations are common industry-wide because they are less expensive and quite adequate for slower speeds and short cable length systems. However, single-ended devices rely on tight termination tolerances. Going beyond this narrow margin of error can cause problems.
In a single-ended configuration, signals on a cable are identified according to the voltage of a single wire (up to 5 volts) relative to a common ground (0 volts). A signal line is recognized as asserted (true) or deasserted (false) according to the strength of its signal and whether it is active high or active low.
Note: All SCSI signals are active low.
Active low signals are deasserted (false) when the signal strength is above a certain voltage, which is typically over 2.0 volts. Active low signals are asserted when the voltage falls below a stated level, which is typically under 0.7 volts. Windows of error may occur if the bus misinterprets a signal that is close to the cutoff.
SCSI buses using the single-ended bus technology provide high-speed communication for short distances. The longer the cable, the more likely errors may be introduced into the signal.
Differential
In a differential cable configuration, the same information is sent with opposite voltages simultaneously through two sets of wires. The information is interpreted by the difference in voltage between the wires. When noise interferes with the signal in this bus configuration, both wires are affected equally. Since the noise level on both wires is the same, the difference in voltage is unchanged. This allows the device to receive information free of noise.
A differential cable configuration determines signals by contrasting the voltage difference between two wires. This has the advantage of allowing cable lengths up to 25 meters (about 80 feet). It is mainly intended for connecting devices externally. Differential configurations are much less subject to signal noise and other termination problems than single-ended configurations.
Differential cabling requires more powerful drivers, which in turn, require additional chips (two for narrow and three for wide). The peripheral device and the SCSI controller card must be designed to use a differential configuration. For these reasons, systems that use a differential electrical specification are less popular than single-ended configurations. However, the need for increased speeds is expanding the popularity of differential configurations.