1. Field of the Invention
This invention relates to digital transmission line termination circuits, and more particularly, to a termination network which combines the functionality of a terminating impedance with protection to the digital transmitter/receiver circuitry, protection from faulty differential receiver output due to its loss of input, and compatibility with various differential receiver technologies.
2. Description of the Prior Art
It is known that termination networks are necessary for digital circuits operating at very high frequencies. High speed logic circuits can produce digital pulses with logic level transitions of very short duration, which in turn can cause reflections or "ringing" on the signal path if these transition times approach the propagation delay of the transmission line. To avoid these effects, termination resistors or termination networks which match the characteristic impedance of the transmission line are placed in parallel with the receiver circuit in order to dissipate the energy of the pulse and prevent reflections or "ringing".
It is also known that "differential" transmitter and receiver circuits have been used to eliminate noise problems on high speed transmitter/receiver interfaces, or on transmitter/receiver interfaces with long transmission lines. A differential transmitter will simultaneously send both a signal and its inverse to the differential receiver, rather than sending only a single signal. Ground voltage differences are therefore nullified between the transmitter circuit and receiver circuit, which provides high noise immunity. The use of differential signals also avoids the problem of voltage drop on the transmission line that is associated with "single-ended" transmission lines. A single-ended transmission line experiences a voltage drop due to the resistance of the transmission line, and this voltage drop decreases the low voltage threshold at the receiver circuit. Differential transmission signals have this voltage drop on both signals, whereby the effect of the voltage drop is cancelled out. A differential approach typically provides more than twice the noise margin of the corresponding single-ended approach.
In order to prevent reflections and ringing on a differential transmission line, the proper characteristic impedance of the differential transmission line must be maintained through the use of a differential termination network. A differential termination network must provide a termination impedance equal to the characteristic impedance of the transmission line at each of the two transmission line signals. It is well known that both signal paths of a high frequency differential transmission must be properly terminated in order to avoid reflections or ringing. There are inherent problems with prior known differential termination networks that occur in a system which requires the differential transmitter circuit to be powered by a voltage source separate from that of the differential receiver circuit and differential termination network.
A differential receiver circuit is designed so that its output signal switches from one logic state to the other when its input signals approach equal voltage potentials. This can create a problem if the transmission line driving the receiver becomes open circuited, which can occur if the transmission line itself is removed or damaged, or if the voltage which supplies the transmitter circuit fails when the receiver circuit is powered by a separate voltage source than that of the transmitter circuit. In a system which utilizes two separate and independent power domains, it is not unlikely that the voltage supplying power to the transmitter can fail, while the receiver remains powered up. The problem which could occur in this instance is that the two inputs of the differential receiver could float to an equal voltage level, thus causing the receiver output to come to rest at an indeterminate voltage level, or to continuously toggle.
The present invention solves the problem at the differential receiver circuit when the differential transmitter circuit signals are not present. In order to ensure that the receiver circuit's output signal does not continuously toggle, a voltage differential must be maintained at the inputs of the receiver circuit upon loss of the transmitter circuit signals. U.S. Pat. No. 4,567,384, by Stuhlmiller, issued Jan. 28, 1986, shows an example of a circuit which may be used with a EIA RS 422/423 receiver to hold the output signal of the receiver to a known output level when the input signals to the receiver become unavailable. The Stuhlmiller design utilizes transistors, resistors, and diodes to create the offset voltage, and it is used in addition to termination impedances. The present invention combines the functionality of a terminating impedance with the ability to provide a voltage differential to hold the receiver circuit in an inactive state upon loss of the transmitter circuit signals, without the need for additional components.
Another problem which has prompted a design feature in this termination network is that different types of differential receiver circuits may require different minimum voltages at the inputs of the receiver circuit. For instance, one type ECL differential receiver may require 2.0 volts minimum at the inputs for the low threshold voltage, while another type may require 2.6 volts minimum at the inputs. As previously stated, if the transmitter circuit fails or the transmitter supply voltage fails, the two input signals to the receiver circuit must be offset so that the it's output does not toggle. Therefore, the two voltages need to be offset from one another, but also may need to maintain a certain minimum voltage in order for the differential receiver circuit to operate correctly. For instance, the differential receiver circuit may require 2.6 volts minimum at the inputs, but the voltage divider termination resistances may only produce 2.2 volts (ignoring the offset voltages). This problem can be solved by providing a threshold voltage within the termination network which increases the two differential receiver input voltages to a minimum threshold voltage. This voltage can then be adjusted according to the minimum required input voltage of the particular differential receiver circuit, without the need to change the resistance values of the termination resistors.
The present invention also embodies a feature which solves a problem where the voltage that powers the differential receiver circuit is separate and independent from the voltage source that powers the differential transmitter circuit. If the voltage source that powers the receiver circuit was to fail, the failed voltage source would appear as a system ground. Since the differential transmitter circuit is expecting the differential receiver voltage to be present, the transmitter circuit could overdrive its DC output current capability when the failed voltage source appears as a system ground. In order to prevent current overdrive of the transmitter circuit upon loss of voltage to the receiver circuit, a resistance is placed between the termination impedance and ground. This separate resistance will effectively raise the equivalent resistance in the termination network when the receiver circuit's voltage has disappeared. The required termination resistances will not be affected by this extra resistance during normal operation however, because a bypass capacitor is used in parallel with the resistor. Therefore, the transmitter circuit is protected when the receiver circuit voltage fails, but during nominal operation the protecting circuitry will be transparent to the rest of the system.
Often Emitter Coupled Logic (ECL) is used for differential transmission because of its high speed characteristics. Although ECL circuitry has the favorable quality of high speed, ECL circuitry consumes a great deal of power. The resistance and bypass capacitor (which protects the transmitter circuit in the event of the receiver circuit's voltage failing) also operates to reduce the overall power consumption of the system during normal operation.
The present invention is intended to alleviate problems where a transmitter circuit and an associated receiver circuit are powered by different voltage sources. The termination network also provides current limiting to reduce power consumption, and provides flexibility in choosing a differential receiver circuit.