Differential TTL
Encyclopedia
Differential TTL is a type of binary electrical
signaling based on the TTL
(transistor
-transistor logic
) standard. 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 on one wire is always the complement of the other. The principle is similar to that of low-voltage differential signaling (LVDS), but with different voltage levels, and even more similar to the RS-422 standard.
Differential TTL is used in preference to single-ended TTL for long-distance signaling. In a long cable, stray electromagnetic field
s in the environment, or stray currents
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, when correctly terminated, is that the differential pair of wires forms 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).
(SSA) standard devised by IBM, but this is mostly obsolete. More efficient signaling techniques such as LVDS are now used instead.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
signaling based on the TTL
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
(transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
-transistor logic
Logic
In philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...
) standard. Normal TTL signals are single-ended, which means that each signal consists of a voltage on one wire, referenced to a system ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
. 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 on one wire is always the complement of the other. The principle is similar to that of low-voltage differential signaling (LVDS), but with different voltage levels, and even more similar to the RS-422 standard.
Differential TTL is used in preference to single-ended TTL for long-distance signaling. In a long cable, stray electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
s in the environment, or stray currents
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
in the system ground, can induce unwanted voltages that cause errors at the receiver. With a differential pair
Differential pair
A differential pair is a pair of conductors used for differential signaling. Differential pairs are usually found on a printed circuit board, in cables , and in connectors...
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, when correctly terminated, is that the differential pair of wires forms 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 ArchitectureSerial Storage Architecture
Serial Storage Architecture is a serial transport protocol used to attach disk drives to servers. It was invented by Ian Judd of IBM in 1990...
(SSA) standard devised by IBM, but this is mostly obsolete. More efficient signaling techniques such as LVDS are now used instead.