Wired-OR connection
A wired-OR-connection ( English for wired OR ), rarely also called phantom-OR-connection, occurs in digital circuit technology when two or more outputs are connected to one another in such a way that the circuit acts like an OR gate . Apart from the actual logic components, no additional electronics are required for this link, apart from a resistor . Open drain outputs are required for the transistor in the output stage in CMOS technology . Open-collector outputs are then required for transistors in bipolar technology .
Generally, this type of logic operation is also called wired logic (or English. As wired logic called). Wired AND technology is directly related to this circuit structure .
Digital components in standard technology
A wired-OR connection can be implemented in positive logic from a wired-AND connection by means of the transformation rules according to de Morgan's laws , as outlined in the adjacent figure. All inputs and the output are inverted. The inverted input signals connected together at the resistor then represent a logical AND link.
From a technical point of view, the wired-OR connection has some disadvantages compared to logic components with standard output stages. With this circuit design, the maximum achievable clock frequency is significantly lower than that of standard logic components. Depending on the logic state, the output transistor in the gate must either be completely conductive (so that a clean L level can be achieved) or must be completely blocked (so that a clean H level can be achieved). To do this, the transistors must be heavily overdriven, which reduces the maximum clock rate that can be achieved during switching processes.
Furthermore, the steepness of the signal edges is limited in this type of circuit. When switching from the H level to the L level, the switching transistor in the gate can switch relatively quickly, since the on-resistance of the conductive transistor is relatively low and thus the parasitic capacitors of the circuit structure can recharge relatively quickly. The situation is different when switching from the L level to the H level. In this case the switching transistors are blocked. In this case, the parasitic capacitors must be recharged through the resistor. However, this is significantly greater than the on-resistance of the transistors and therefore has the effect that the edge rise of the output signal when switching from L to H is significantly slower than when switching from H to L. Due to the different edge rate of the output signal, it is strongly recommended that a component with a Schmitt trigger input is connected downstream. However, additional components have the further disadvantage that additional signal propagation times arise.
All transistors have minimal conductivity in the form of collector-emitter residual currents even when they are blocked. The greater the number of transistors connected in parallel, the greater the sum of the residual currents. These residual currents flow through the resistor and can lead to a distortion of the logic level if the level is H. Due to this fact, the resistance must be relatively low so that clean logic levels can be set despite the parallel connection of the outputs of several gates. A low-resistance resistor, however, results in a higher power consumption and thus a higher power loss of the circuit.
In the engineering-based circuit development in standard logic, the wired-OR connection is practically no longer used in new developments.
Digital components in ECL technology
Another circuit variant in the emitter-coupled logic (ECL) uses the direct possibility of creating a wired-OR connection by interconnecting open emitter outputs. The advantage of the direct link is that you save inverter stages and the associated additional processing times. Due to the differential signals in ECL technology, two resistors are required. In the classic transistor-transistor logic , at least two additional diodes after the output drivers in addition to the resistor are required for the direct implementation of a wired OR link .
With ECL technology, the two load resistors are part of the circuit structure. The ECL technology in general is only reserved for special applications.
Analog comparators with digital switching outputs
Analog comparators are components that compare either a voltage value with a reference voltage or two voltage values with one another. A comparison between larger and smaller, within a defined voltage window or outside of a defined voltage window is possible. These comparators often have digital outputs, which, depending on the design, can also be designed as open-drain or open-collector outputs. If several comparators have to be logically linked with one another in this technology, this can also be done in wired-OR technology. In the case of purely analog circuits, wired-OR operations make it possible to completely dispense with digital logic components, depending on the circuit structure.
literature
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 12th edition. Springer, 2002, ISBN 3-540-42849-6 .
- Klaus Beuth: Digital technology . 10th edition. Vogel, 1998, ISBN 3-8023-1755-6 .
- Manfred Seifart, HelmutBeikirch: Digital circuits . 5th edition. Technology, 1998, ISBN 3-341-01198-6 .