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Wired logic connection
Wired logic connection
Wired logic connection
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A wired logic connection is a logic gate that implements boolean algebra (logic) using only passive components such as diodes and resistors. A wired logic connection can create an AND or an OR gate. Limitations include the inability to create a NOT gate, the lack of amplification to provide level restoration, and its constant ohmic heating for most logic (particularly more than CMOS) which indirectly limits density of components and speed.

Wired logic works by exploiting the high impedance of open collector outputs (and its variants: open emitter, open drain, or open source) by just adding a pull-up or pull-down resistor to a voltage source, or can be applied to push-pull outputs by using diode logic (with the disadvantage of incurring a diode drop voltage loss).

Active-high wired AND connection

[edit]

See also: Diode logic § Active-high AND logic gate

Open-collector buffers connected as wired AND.

The wired AND connection is a form of AND gate. When using open collector or similar outputs (which can be identified by the ⎐ symbol in schematics), wired AND only requires a pull up resistor on the shared output wire. In this example, 5V is considered HIGH (true), and 0V is LOW (false). This gate can be easily extended with more inputs.

When all inputs are HIGH, they all present high impedance, and the pull-up resistor pulls output voltage HIGH, but if any input is LOW, they pull the output LOW:[1]

Inputs Output
A B A AND B
HIGH LOW LOW
LOW HIGH LOW
LOW LOW LOW
HIGH HIGH HIGH

When driving a load, the HIGH output is reduced by the pull-up's voltage drop, though the LOW output is almost 0V. But if diode logic is used, each input requires a diode, and the LOW output voltage will additionally be raised by the diode's forward voltage. Care should be taken to ensure the output still lies within valid voltage levels.

Active-high wired OR connection

[edit]
Open-emitter buffers connected as wired OR.

See also: Diode logic § Active-high OR logic gate

The wired OR connection electrically performs the Boolean logic operation of an OR gate using open emitter or similar inputs (which can be identified by the ⎏ symbol in schematics) connected to a shared output with a pull-down resistor. This gate can also be easily extended with more inputs.

When all inputs are LOW, they all present high impedance, and the pull-down resistor pulls the output voltage LOW, but if any input is HIGH, they pull the output HIGH:

Inputs Output
A B A OR B
LOW LOW LOW
LOW HIGH HIGH
HIGH LOW HIGH
HIGH HIGH HIGH

When driving a load, the LOW output is raised by the pull-down's voltage drop, though the HIGH output is almost the supply voltage (5V). But if diode logic is used, each input requires a diode, and the HIGH output voltage will additionally be lowered by the diode's forward voltage.

Reversing active level

[edit]

An active-high wired AND can be treated as active-low wired OR (and an active-high wired OR can be treated as active-low wired AND), by using active-low logic (or negative logic) and applying De Morgan's laws.

Compatibility of wired AND OR using diodes

[edit]
Main article: Diode logic
Wired AND in diode logic.
Wired OR in diode logic.

Diode logic uses a diode for each input in addition to a shared pull-up resistor (for wired AND) or a pull-down resistor (for wired OR). However, each stage of diode logic reduces output voltage levels. So without amplification, the output voltage may not be compatible with the primary logic family.


References

[edit]
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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Wired logic connection

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from Grokipedia
A wired logic connection is a method in digital electronics for implementing Boolean logic functions, such as AND or OR gates, by directly interconnecting the outputs of multiple logic gates—typically open-collector or open-drain types—with a shared pull-up or pull-down resistor, or alternatively using only passive components like diodes and resistors without active elements. This approach leverages the electrical properties of the connected components to perform logic operations, enabling efficient multi-input configurations without requiring dedicated gate ICs for every function. In the passive variant, known as , a wired-AND gate is formed using a connected to the supply voltage and diodes for each input; the output remains high (logic 1) only when all are high, as any low input forwards the diode to ground the output. Similarly, a wired-OR gate employs a and diodes, producing a high output if at least one input is high. These passive implementations, however, lack voltage level restoration and cannot realize NOT functions, limiting their use to simple . In active wired logic, prevalent in TTL and CMOS technologies, open-collector outputs from gates like NAND or NOR are tied together with a single external pull-up resistor to create a wired-AND function (or wired-OR for active-low signals), preventing output conflicts and allowing higher fan-in than standard gates. This configuration was historically significant in early integrated circuits, such as the 7400-series TTL family, where it facilitated bus architectures, priority encoding, and interrupt systems by enabling multiple devices to share a signal line without contention. Open-drain variants in modern CMOS designs extend this to lower-power applications, including I²C buses and wired-OR interrupt lines. Beyond traditional uses, wired logic connections have seen resurgence in specialized low-power processors, such as all-digital asynchronous designs for , where they minimize by avoiding complex and storage elements. For instance, wired-logic architectures in field-programmable gate arrays (FPGAs) achieve up to 47 times greater energy efficiency compared to conventional binary processors for tasks like image classification. Despite advantages in simplicity and scalability, wired logic requires careful selection to manage loading and speed, and it is prone to issues like increased propagation delay in large networks.

Fundamentals

Definition and Basic Principles

Wired logic connection is a technique in digital electronics for implementing basic logic functions, such as AND or OR, by directly coupling multiple outputs—typically open-collector in bipolar technologies or open-drain in MOS—to a shared signal line, relying on a pull-up or pull-down rather than additional active gating elements. This method exploits the high-impedance state of the outputs when inactive, allowing the shared line to reflect the collective behavior of all connected signals without requiring additional logic hardware. Originating in early -transistor logic (TTL) families during the 1960s, wired logic enables efficient multi-device interfacing on buses while minimizing component count. A purely passive form of wired logic, known as diode logic, uses diodes and resistors without active transistors to implement AND or OR functions. In diode-AND, diodes connect inputs to the output with a pull-up resistor; the output is high only if all inputs are high. However, it lacks level restoration and inversion capability. At its core, open-collector outputs feature a transistor (e.g., NPN in TTL) where the collector terminal is exposed externally, allowing it to sink current to ground when active but remaining in a high-impedance state when off, unable to source current. A pull-up resistor connected between the shared line and the positive supply voltage (VCC) ensures the line defaults to a logic high when all outputs are inactive, preventing floating states and electrical contention that could arise if multiple devices attempted to drive the line simultaneously. Similarly, open-drain outputs in CMOS use an NMOS transistor to pull low, with the same pull-up requirement; pull-down resistors serve the analogous role in sourcing configurations. This setup avoids damage from conflicting drives, as only sinking (or sourcing) occurs, with the resistor defining the opposite level. In a wired-AND configuration, typically using active-low signals and a , the shared line is pulled low by any active (low) output, remaining high only if all connected outputs are inactive (high-Z), thus performing an implicit AND operation on the active-low . For example, connecting outputs of two open-collector NAND gates (where each NAND output goes low only if its are all high) can implement more complex AND functions, but for simple illustration with inverters, the wired connection of open-collector inverters performs a NOR: the line is high only if all are low. The for two open-collector inverters (active-low outputs) is:
Input AInput BInverter A OutInverter B OutWired Output
Low (0)Low (0)High-ZHigh-ZHigh (1)
Low (0)High (1)High-ZLow (0)Low (0)
High (1)Low (0)Low (0)High-ZLow (0)
High (1)High (1)Low (0)Low (0)Low (0)
Conversely, a wired-OR configuration employs active-high signals with a pull-down resistor, where any active (high) output pulls the line high, keeping it low only if all outputs are inactive (low), implementing an OR function. A basic circuit consists of the shared bus line connected to multiple transistor collectors (or drains), with the pull-up (or pull-down) resistor tied to the appropriate supply rail, forming a simple wired network without diodes or additional components in standard open-collector setups. Active-high and active-low refer to the polarity of assertion—active-high signals are driven high to activate, while active-low are driven low—dictating the resistor choice and logic interpretation.

Historical Development

The concept of wired logic connections traces its roots to early digital logic families, particularly diode-transistor logic (DTL) developed in the late and early , where diode networks performed basic AND functions, often requiring pull-up or pull-down resistors to establish logic levels on shared lines. These configurations served as precursors to true wired logic by enabling simple multi-input gating through passive diode steering, though they relied on active transistors for inversion and were limited by diode forward voltage drops and constraints. DTL's use of resistor-terminated arrays in early integrated circuits laid the groundwork for more efficient transistor-based implementations, distinguishing it from purely passive resistor-transistor logic (RTL) by improving noise margins. Wired logic emerged prominently in the 1960s with the advent of transistor-transistor logic (TTL), invented in 1961 by James L. Buie at TRW Inc., which introduced open-collector outputs to facilitate multi-drop buses without active pull-up transistors. This innovation allowed multiple outputs to share a signal line, with a single external pull-up resistor enabling wired-AND (active-low) or wired-OR (active-high) functions by exploiting the high-impedance state when outputs were not sinking current. Texas Instruments popularized the approach through its SN54/74 series TTL devices, introduced in the mid-1960s with the ceramic SN5400 in 1964 and the cost-effective plastic SN7400 series in 1966, which included open-collector variants like the SN7406 for interfacing and bus applications. A key milestone was its adoption in early microprocessors, such as the Intel 8080 released in 1974, where open-collector interrupts on the INT pin supported wired-OR configurations for priority handling among peripherals. In the , wired logic evolved into CMOS equivalents with open-drain outputs, adapting the principle to lower-power integrated circuits while maintaining compatibility with TTL levels. Semiconductors (now NXP) developed the bus protocol in 1982, employing open-drain drivers on SDA and SCL lines with pull-up resistors to realize wired-AND arbitration and multi-master synchronization, which became a standard for short-distance inter-chip communication. This shift addressed TTL's higher power dissipation, enabling widespread use in consumer electronics and embedded systems. The pure form of wired logic began to decline in the with the proliferation of programmable logic devices, particularly field-programmable gate arrays (FPGAs) introduced by in 1985, which integrated configurable logic blocks and routing to replace discrete wired connections with reconfigurable on-chip interconnects. By the mid-, FPGA capacity had grown exponentially, reducing reliance on external wiring for custom logic while offering greater flexibility and density. Despite this, wired logic principles persisted in specialized embedded applications, such as bus protocols like , where open-drain configurations remain essential for low-cost, multi-device interfacing.

Core Implementations

Active-High Wired AND Connection

The active-high wired AND connection involves connecting multiple open-collector outputs (in bipolar technology) or open-drain outputs (in ) to a common node, with a attached from the node to the positive supply voltage Vcc. This setup allows the common node—serving as the logic output—to attain a logic high state (near Vcc) solely when all connected outputs remain in their high-impedance (off) state, preventing any current sink to ground; the maintains the high level in this condition. Activation of any single output causes it to sink current through the , forcing the node to logic low (near ground). Signal propagation in this configuration proceeds as follows: each contributing drives its output such that the turns on (sinking current and pulling the node low) when the corresponding input is in the low state, and remains off (high-impedance) when the input is high. Consequently, if all inputs are high, every is off, and the holds the output high. However, if any input transitions to low, its associated activates, sinking current and driving the output low immediately, regardless of the states of other inputs. This ensures the overall output reflects the AND operation, being high only when no is conducting. For an n-input active-high wired AND, the demonstrates that the output is logic 1 exclusively when all inputs are logic 1; any low input results in a logic 0 output. The generalized form is:
Input CombinationOutput
At least one input = 00
All inputs = 11
For a concrete 2-input case:
ABOutput (A AND B)
000
010
100
111
This table assumes each input controls a gate configured to sink current precisely when that input is low. In TTL implementations, open-collector variants such as the 7407 hex buffer/driver are commonly employed, where the outputs of multiple 7407 gates are tied together at the common node with an external (typically 1 kΩ to 4.7 kΩ). The 7407 is a non-inverting buffer, sinking current when its input is low, which enables the collective AND behavior across several gates. Fan-out limitations in active-high wired AND connections arise primarily from resistor loading effects on voltage levels and switching speed, as well as the current-handling capabilities of the outputs. The value must balance providing sufficient high-state voltage while allowing adequate sink current without excessive power dissipation or voltage droop. A key constraint is the maximum number of inputs (n), often determined by ensuring the active sinker(s) can handle the pull-up current I_pull = Vcc / (e.g., 5 V / 2.2 kΩ ≈ 2.3 mA) without exceeding V_OL max, and considering total sink current when multiple gates are active. For standard TTL open-collector outputs with I_OL max ≈ 16 mA (e.g., 7407), practical designs support n up to 10–20 inputs before margins degrade, depending on the series (e.g., lower for at ~8 mA) and loading effects like bus that increase propagation delay.

Active-High Wired OR Connection

The active-high wired OR connection implements OR logic in environments where a high voltage level represents logic 1, typically using open-emitter outputs from bipolar transistors, such as those in (ECL) families. In this configuration, multiple open-emitter outputs are tied together to form a common bus line, with a pull-down (typically 50 Ω) connected from the bus to V_EE (the negative supply, logic 0). Each output stage consists of an emitter-follower whose collector is tied to the positive supply (Vcc), allowing the emitter to source current and pull the bus high when active, while remaining open () when inactive. This setup ensures the bus remains low due to the pull-down unless at least one connected device asserts its output. The behavior of the circuit follows standard OR logic: the output is logic 0 only when all inputs are 0, and transitions to logic 1 as soon as any input asserts 1, with the asserting emitter follower driving the bus voltage toward Vcc minus a base-emitter drop (typically around 0.8 V swing in ECL). This wired connection leverages the low of the emitter followers for fast rise times during assertion, while the shared bus enables high without additional gating delay. In contrast to pull-up based setups like active-high wired-AND, this pull-down approach avoids contention when multiple outputs assert simultaneously, as all active emitters source current in parallel. The effective for a two-input active-high wired OR is as follows:
Input AInput BOutput
000
011
101
111
A representative example is found in high-speed ECL SRAM designs, where wired-OR connections combine address predecoder signals; for instance, in a 1 Mb ECL BiCMOS SRAM, this technique achieves 5 ns access times by paralleling open-emitter outputs to OR multiple select lines with near-zero added delay. Regarding immunity, the fall time to logic 0—governed by the pull-down and bus RC constant—can introduce slower transitions in capacitive loads, potentially reducing margin in noisy environments, though ECL's typical margins of 0.325 V (high-to-low) and 0.335 V (low-to-high) provide robustness for speeds up to gigahertz.

Advanced Variations

Reversing Active Levels

In wired logic connections, reversing the active levels allows adaptation of the circuit's polarity to meet specific , such as converting an active-low configuration to active-high or vice versa. One primary method involves using complementary pull resistors to alter the default state of the shared line. A connected to the positive supply establishes an active-low wired-OR configuration, where the line remains high unless any open-collector output pulls it low, implementing an OR function for low-active signals. Conversely, a pull-down resistor to ground enables an active-high wired-AND setup, where the line stays low by default and requires all outputs to be inactive for it to rise, though this demands careful selection of output types capable of sourcing current, as standard NPN open-collector TTL cannot directly pull high. Another approach to reversing active levels employs inverter gates at the inputs or outputs to flip the signal polarity before or after the wired connection. For instance, the 7404 hex inverter IC can be inserted in the signal path, inverting the logic levels with a typical delay of 10 ns per gate in standard TTL. This method is particularly useful when interfacing mismatched logic families or when the base wired configuration needs inversion without altering the pull resistor setup. Open-collector variants like the 7405 can be used for the inverters to maintain compatibility with wired nodes, supporting higher voltage swings up to 15 V. Active-low wired-AND configurations are prevalent in TTL circuits using open-collector outputs with a , where the shared line defaults to high and is pulled low only if all connected inputs are low, effectively performing a NAND operation but functioning as an AND for low-active signals. This setup leverages the current-sinking capability of TTL open-collector gates, such as those in the 7401 NAND or 74156 decoder, to create multi-input logic without additional gates. Reversing levels via inverters introduces additional propagation delay, typically adding 10 ns per inversion stage in TTL, which can accumulate in multi-stage designs and affect overall circuit timing compared to direct wired connections without reversal. For example, consider a two-input active-low OR implemented with open-collector buffers and a ; to convert this to an active-high AND, double inversion is applied—first by inverting both inputs before feeding them into the wired node (turning the OR of inverted signals into an AND of originals), and second by inverting the wired output. This results in the desired active-high AND behavior while preserving the wired topology. Despite these techniques, reversing active levels can introduce pitfalls, including increased power consumption if both pull-up and pull-down resistors are inadvertently used simultaneously, leading to a steady current draw across the resistor divider (e.g., several mA with 2.2 kΩ resistors at 5 V). Additionally, improper resistor selection or absence of pulls may cause floating states on the wired line, resulting in undefined logic levels and potential metastability in downstream gates.

Diode-Based Compatibility for AND and OR

Diode-based wired logic enables the implementation of AND and OR functions through passive components, where act as unidirectional switches to enforce logical behavior on a shared bus. In a configuration, multiple are connected in parallel with their cathodes to the individual and anodes tied to a common output bus, accompanied by a to the positive supply voltage. The output goes high only when all are high, as each remains reverse-biased, allowing the to charge the bus; if any is low, its corresponding forward-biases and pulls the bus low. For a OR configuration, are arranged in parallel with anodes to the inputs and cathodes tied to the output bus, paired with a pull-down to ground. The output asserts high if at least one input is high, as the forward-biased from that input drives the bus high against the pull-down; with all inputs low, the pull-down keeps the bus low. The for the OR output can be approximated as Vout≈max⁡(Vin)−VfV_{out} \approx \max(V_{in}) - V_f, where VfV_f is the forward voltage, set by the highest input. In hybrid setups, diodes facilitate compatibility between AND and OR sections by preventing reverse current flow, or backfeed, in mixed-logic environments such as address decoding circuits. For instance, series-connected diodes in an AND stage can feed into parallel diodes of an OR stage on the bus, ensuring that a low from the AND does not erroneously activate the OR, while allowing qualified signals to propagate. This diode isolation was employed in early computers like the PDP-8, where wired diode gates expanded logic functions for memory addressing and control without active buffering. The forward voltage drop across diodes introduces challenges, typically 0.7 V for diodes, which diminishes margins in the logic levels; to mitigate this, Schottky diodes with a lower drop of approximately 0.3 V are often used, preserving in chained . Despite these benefits, diode-based wired logic incurs higher component costs due to the need for multiple s per gate and suffers from added junction capacitance, which increases propagation delay—particularly rise times—through RC charging effects on the bus.

Practical Considerations

Applications in Digital Circuits

Wired logic connections are widely employed in bus mechanisms within digital circuits, enabling multiple devices to share signal lines without contention. A prominent example is the system of the microprocessor, where pins such as RST 7.5, RST 6.5, RST 5.5, and INTR utilize open-collector outputs to form a wired-OR configuration. This setup allows peripheral devices to pull the shared line low to assert an , with a restoring the high state when idle, ensuring the processor detects any active request efficiently. In address decoding for memory expansion, wired-AND configurations simplify chip select generation by connecting multiple open-collector gates in parallel. For instance, high-order address bits from the processor are fed into NAND gates whose outputs drive the common line; the line remains high (inactive) unless all relevant address bits align to deassert the gates simultaneously, selecting the target device. This approach minimizes decoding complexity in systems with multiple memory modules. Communication protocols like and SMBus leverage open-drain drivers to implement wired-AND logic for multi-master on the serial data line (SDA). When multiple masters transmit simultaneously, any device attempting to drive a '1' (high) while another drives '0' (low) detects the discrepancy via the bus voltage and cedes control, preventing . The clock line (SCL) similarly uses this mechanism for , supporting robust bus sharing in and peripheral networks. Legacy systems from the , such as TTL-based minicomputers like the DEC PDP-11, relied on wired logic for shared buses, using open-collector TTL gates (e.g., 74LS series) to handle interrupts and control signals across multiple boards. In contemporary niche applications, field-programmable gate arrays (FPGAs) emulate wired logic through configurable open-drain I/O pins, facilitating interfaces with legacy protocols or low-power sensor networks. For example, / FPGAs use tri-state buffers (e.g., OBUFT ) to replicate open-drain behavior on GPIO, allowing shared lines for event signaling among battery-constrained sensors, where only active devices pull low to minimize power draw.

Advantages, Disadvantages, and Limitations

Wired logic connections offer several advantages over traditional active gate logic in digital circuits, primarily through reduced component requirements. By connecting multiple open-collector outputs with a single , wired-AND or wired-OR functions can be implemented without additional gates, leading to a lower overall component count and simplified board layouts, which is particularly beneficial for high-density designs. This approach can result in approximately 50% fewer transistors in systems requiring high , as it avoids the need for multi-input gates. Additionally, the inherent multi-drop capability supports bus architectures, enabling efficient signal distribution across multiple devices without extra buffering hardware, thereby reducing costs in applications like address decoding. Despite these benefits, wired logic suffers from notable disadvantages related to and . Switching speeds are slower compared to standard totem-pole outputs due to the RC time constants introduced by the and bus , where the trise=Rpull×Cbust_{rise} = R_{pull} \times C_{bus} can extend propagation delays by 2-5 times for configurations with large numbers of inputs. is also limited, typically supporting only 10-20 TTL loads before signal integrity degrades, as each additional connection increases loading on the shared line. Power consumption is higher due to the continuous current through the always-on , even when the output is low, exacerbating inefficiency in static conditions. Key limitations further constrain the use of wired logic in modern designs. Long lines are prone to pickup and reflections, necessitating termination resistors to maintain , which adds complexity. Direct incompatibility with push-pull (totem-pole) outputs requires modifications like open-collector configurations to prevent damage from conflicting drive currents. Consequently, wired logic should be avoided in high-speed applications exceeding 100 MHz, where RC delays become prohibitive, or in low-power environments, where tri-state buffers provide similar multi-drop functionality with better efficiency. Mitigation strategies include using buffered drivers to amplify signals or active pull-ups to reduce RC effects, though these partially offset the simplicity gains.

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