1. Field of the Invention
This invention is related to high speed digital signal transmission buses, or nets, and more specifically to the transmission of digital signals from multiple sources that are implemented in emitter-coupled-logic (ECL) that employs multi-chip modules with thin film interconnect.
2. Description of Background Art
For many years standard techniques for driving high-speed signals in digital computers have been employed. One standard approach is to match the impedance of the transmission line to the driver's ability to drive output current and the system's ability to carry this current and dissipate the power. For emitter-coupled logic (ECL) the standard impedance is 50 to 75 ohms. This value is chosen to allow the driver to be of a reasonable physical size, and the effect of loading capacitance on the transmission line to be minimized. For example, driving a 50 ohms impedance with a 0.8 volt swing from low to high or high to low causes a change in current of about 16 milliamperes. If this current change is not absorbed by a termination resistor at the end of the net, or otherwise clamped off or dissipated, it will reflect back toward the driver as an unwanted signal of increased magnitude.
Standard practice in printed circuit boards, backpanels, cables and similar transmission media to avoid this problem is to place a resistor at the end of the wire whose value equals the impedance of the transmission line. This is called end termination, or parallel termination. Such termination works satisfactorily for wires with controlled impedance and with a low value of resistance in series between the driver and the termination resistance. Any other resistance in series either due to high resistance metal, or other added resistance will cause a voltage divider to exist between the driver and the receiver which is very undesirable with ECL since the allowable drop in voltage is only about 135 millivolts before the receiver is unable to determine if a proper signal has arrived.