Data bus using open drain drivers and differential receivers together with distributed termination impedances

An electronic system including a plurality of circuit board modules, wherein each module is interconnected to the other modules via a data bus consisting of a plurality of conductive paths, the data bus of the type which makes use of distributed termination impedances, and wherein each module comprises:

• operating logic circuits, mounted on the module, which provide a plurality of transmit logic signals to be communicated to the logic circuits located on the other modules, and which also receive a plurality of receive logic signals from the logic circuits located on the other modules;
• a multiplexor, connected to receive an output enable signal, to receive one of the transmit logic signals from the operating logic circuits, and a deasserted logic signal, for providing the transmit logic signal as a multiplexor output signal when the output enable signal is asserted, and for providing the deasserted logic signal as a multiplexor output signal when the output enable signal is deasserted;
• a clock generator, for generating a clock signal;
• a driver flip flop, connected to receive one of the transmit logic signals from the operating logic circuits at a data input and the clock signal, and to provide a latched transmit logic signal;
• a direct current (DC) power supply circuit, located on the module, for providing a termination voltage to the logic circuits mounted on the same module;
• an open drain driver transistor, connected to receive the latched transmit logic signal, and connected to one of the conductive paths, to thereby forward the latched transmit logic signal to another module;
• a termination impedance, mounted on the module, and connected between the conductive path and the termination voltage, such that the conductive path has a single termination impedance connected thereto in each module, the resistance of the termination impedance being chosen so that a minimum propagation delay is provided on the conductive path both when the driver transistor in a given module drives a logic signal consisting of alternating logic levels in succession, as well as when the driver transistors in two different modules drive a given logic level in succession;
• a reference voltage circuit, connected to receive the termination voltage, and to generate a reference voltage;
• a differential receiver, connected to receive the reference voltage and connected to one of the conductive paths, for receiving a receive logic signal from one of the other modules, and to thereby provide a differential logic signal;
• and
• a receiver latch, connected to the differential receiver and the clock signal, to provide a latched receive logic signal to the operating logic circuits.

A bus node for connecting to other bus nodes via a data bus consisting of a plurality of conductive paths, the data bus of the type which uses distributed termination impedances, wherein each bus node comprises:

• an open drain driver transistor, connected to one of the conductive paths;
• means for providing a bus termination voltage;
• a termination impedance, located at the bus node, and connected between the conductive path and the termination voltage, such that each conductive path has a single termination impedance connected thereto at each bus node;
• a reference voltage circuit, connected to the termination supply voltage, and to generate a reference voltage;
• and
• a differential receiver, connected to receive the reference voltage and connected to one of the conductive paths.

A method for sending data signals between modules in an electronic system over a data bus consisting of a plurality of conductive paths, the data bus of the type which uses distributed termination impedances, the method comprising the steps of, in each module:

• providing a plurality of transmit logic signals to be communicated to other modules;
• generating a direct current (DC) termination voltage;
• forwarding the transmit logic signal to one of the other modules by connecting an open drain driver transistor between the transmit logic signal and one of the conductive paths;
• connecting a termination impedance between the conductive path and the termination voltage, such that each conductive path has a single termination impedance connected thereto in each module;
• generating a local reference voltage from the termination voltage;
• and
• detecting a receive logic signal by comparing the local reference voltage to the voltage on one of the conductive paths.

A method of operating a open drain driver and differental receiver in a bus node, wherein the bus node is connected to a bus making use of distributed termination impedances, the method comprising the steps of:

• connecting the open drain driver to one of the conductive paths;
• providing a bus termination voltage in each bus node;
• connecting a termination impedance between each conductive path and the termination voltage, such that each conductive path has a single termination impedance connected thereto at each bus node;
• generating a local reference voltage from the termination voltage;
• and
• detecting a receive logic signal by comparing the local reference voltage to the voltage on one of the conductive paths.