Differential TTL is a type of binary

electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...

signaling based on the transistor-transistor logic (TTL) concept. Standards implementing differential TTL include

RS-422 RS-422, also known as TIA/EIA-422, is a technical standard originated by the Electronic Industries Alliance that specifies electrical characteristics of a digital signaling circuit. It was meant to be the foundation of a suite of standards that ...

. It enables electronic systems to be relatively immune to noise. Normal TTL signals are single-ended, which means that each signal consists of a voltage on one wire, referenced to a system ground. The "low" voltage level is zero to 0.8 volts, and the "high" voltage level is 2 volts to 5 volts. A differential TTL signal consists of two such wires, also referenced to a system ground. The

logic level In digital circuits, a logic level is one of a finite number of states that a digital signal can inhabit. Logic levels are usually represented by the voltage difference between the signal and ground, although other standards exist. The range ...

on one wire is always the complement of the other. The principle is similar to that of

low-voltage differential signaling Low-voltage differential signaling (LVDS), also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard. LVDS operates at low power and can run at very high speeds ...

(LVDS), but with different voltage levels. Differential TTL is used in preference to single-ended TTL for long-distance signaling. In a long cable, stray electromagnetic fields in the environment, or stray

currents Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

in the system ground, can induce unwanted voltages that cause errors at the receiver. With a differential pair of wires, roughly the same unwanted voltage is induced in each wire. The receiver subtracts the voltages on the two wires, so that the unwanted voltage disappears, and only the voltage created by the driver remains. A second advantage of differential TTL is that the differential pair of wires can form a current loop. The driver sources a current from the power supply into one wire. This current passes along the wire to the receiver, through the termination resistor and back up the other wire, then back through the driver and down to ground. No net current is exchanged between the driver and receiver, which means that none of the signal current has to return through the ground connection (if there is one) between the two ends. This arrangement prevents the signal from injecting currents into the ground connection, which might upset other circuits attached to it. Differential TTL is the most common type of high-voltage differential signaling ( HVDS).

Applications

Differential TTL signaling was used in the

Serial Storage Architecture Serial Storage Architecture (SSA) was a serial transport protocol used to attach disk drives to server computers. History SSA was invented by Ian Judd of IBM in 1990. IBM produced a number of successful products based upon this standard before ...

(SSA) standard devised by IBM, but this is mostly obsolete. More efficient signaling techniques such as

LVDS Low-voltage differential signaling (LVDS), also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard. LVDS operates at low power and can run at very high speeds ...

are now used instead.

References

{{reflist Logic families

Applications

See also

References