Community Hub0 Subscribers
Community hub
Multidrop bus
View on Wikipediafrom Wikipedia
A multidrop bus (MDB) is a computer bus able to connect three or more devices. A process of arbitration determines which device sends information at any point. The other devices listen for the data they are intended to receive.
Multidrop buses have the advantage of simplicity and extensibility, but their differing electrical characteristics make them relatively unsuitable for high frequency or high bandwidth applications.
In computing
[edit]Since 2000,[citation needed] multidrop standards such as PCI and Parallel ATA are increasingly being replaced by point-to-point systems such as PCI Express and SATA. Modern SDRAM chips exemplify the problem of electrical impedance discontinuity.[clarification needed] Fully Buffered DIMM is an alternative approach to connecting multiple DRAM modules to a memory controller.
For vending machines
[edit]MDB/ICP
[edit]MDB/ICP (formerly known as MDB) is a multidrop bus computer networking protocol used within the vending machine industry, currently published by the American National Automatic Merchandising Association.
ccTalk
[edit]The ccTalk multidrop bus protocol uses an 8 bit TTL-level asynchronous serial protocol. It uses address randomization to allow multiple similar devices on the bus (after randomisation the devices can be distinguished by their serial number). ccTalk was developed by CoinControls, but is used by multiple vendors.
See also
[edit]External links
[edit]| General | |
|---|---|
| Standards |
|
| Storage |
|
| Peripheral | |
| Audio | |
| Portable | |
| Embedded | |
Interfaces are listed by their speed in the (roughly) ascending order, so the interface at the end of each section should be the fastest.  Category | |
Multidrop bus
View on Grokipediafrom Grokipedia
Fundamentals
Definition and Principles
A multidrop bus is a communication architecture in which multiple devices, or nodes, are connected to a single shared transmission medium, enabling data exchange typically between a central master device and one or more slave or peer devices.[5] This shared medium allows all connected nodes to potentially access the bus simultaneously, distinguishing it from dedicated connections.[6] The fundamental principles of a multidrop bus revolve around a common transmission line, often implemented as a single wire, twisted-pair cable, or similar conductor for carrying data and sometimes clock signals.[7] Communication is generally half-duplex, meaning data flows in one direction at a time over the shared medium to avoid conflicts, with nodes taking turns to transmit or receive.[8] To maintain signal integrity, electrical characteristics such as impedance matching are critical; termination resistors at the ends of the bus match the cable's characteristic impedance to prevent reflections that could distort signals.[6][7] In terms of topology, a multidrop bus typically employs a linear or daisy-chain configuration, where nodes are attached along the length of the shared medium, with terminators placed at both ends to absorb signals and minimize interference.[9] This contrasts with point-to-point topologies, which use dedicated lines between individual pairs of devices without sharing the medium.[10] Multidrop buses can operate in serial mode, transmitting bits sequentially over the shared line, or parallel mode, sending multiple bits simultaneously via separate lines; additionally, they support synchronous operation, where a clock signal coordinates timing, or asynchronous modes that rely on start/stop bits for synchronization.[7][11]Historical Development
The concept of the multidrop bus emerged in the late 1960s and early 1970s as computing shifted from large mainframes to more accessible minicomputers, enabling cost-effective connections for multiple peripherals on a shared medium. IBM's System/360, introduced in 1964, influenced bus designs through its standardized I/O channel architecture, which laid groundwork for parallel data transfer concepts later adapted in multidrop configurations. By 1970, Digital Equipment Corporation (DEC) implemented the UNIBUS in its PDP-11/20 minicomputer, a classic multidrop asynchronous bus supporting up to 20 devices per segment for DMA transfers and interrupts, significantly reducing wiring complexity and costs for peripherals in laboratory and industrial settings.[12][13] In the 1980s, multidrop buses proliferated with the rise of personal computing, balancing expandability and affordability. Apple's NuBus, introduced with the Macintosh II in 1987 and based on the IEEE 1196 standard finalized that year, provided a platform-independent, synchronous multidrop architecture with automatic configuration, allowing up to six expansion cards for graphics and networking in desktop systems.[13] Similarly, IBM's Industry Standard Architecture (ISA) bus, originating with the 1981 IBM PC as an 8-bit multidrop interface and extended to 16 bits in the 1984 PC/AT, enabled widespread peripheral adoption by clone manufacturers, supporting devices like hard drives and sound cards through shared address and data lines.[13][14] The 1990s and 2000s marked a transition to serial multidrop protocols for improved noise immunity and scalability in embedded applications. Philips Semiconductors (now NXP) developed the I²C bus in 1982 as a two-wire serial multidrop interface for inter-chip communication, but its adoption surged in the 1990s for consumer electronics like TVs and sensors due to low pin count and multi-master support. In automotive systems, Robert Bosch GmbH patented the Controller Area Network (CAN) in 1986, introducing a robust serial multidrop bus with non-destructive arbitration; by the 1990s, it became standard for vehicle ECUs, evolving to higher speeds like CAN FD in the 2010s. Meanwhile, Modicon's Modbus, launched in 1979 as a serial multidrop protocol for PLCs, gained prominence in the 1990s for industrial automation and continued evolving with variants like Modbus TCP for Ethernet integration.[15][16][17] Post-2010, multidrop buses integrated deeply into IoT and embedded systems, leveraging legacy protocols for low-power, distributed sensing. I²C and CAN found renewed use in smart devices and automotive networks, while Modbus persisted in industrial IoT gateways as of 2025, supporting remote monitoring with minimal overhead. This evolution reflects a focus on reliability in resource-constrained environments, with standards bodies like SAE and IEC ensuring backward compatibility.[16][17]Technical Operation
Addressing and Communication Mechanisms
Multidrop bus systems encompass both serial and parallel topologies, with addressing and communication mechanisms varying accordingly. In serial multidrop buses, addressing schemes rely on unique node identifiers embedded in messages to target specific devices among multiple connected nodes. Typically, these identifiers use fixed-length binary codes, such as 7-bit addresses that support up to 128 unique nodes (excluding reserved addresses) or 10-bit addresses that expand the range to over 1,000 nodes for larger networks.[15] For instance, in the I²C protocol, the 7-bit scheme places the address in the most significant bits of the first byte following the start condition, followed by a read/write bit, while the 10-bit scheme employs a two-byte sequence starting with a special prefix (11110XX).[15] Broadcast addressing, often implemented via a reserved all-zeroes pattern like the general call address (0000000 in I²C), allows a message to reach all nodes simultaneously without specifying an individual identifier.[15] In parallel multidrop buses, such as PCI and SCSI, addressing typically involves a shared address bus where devices decode specific ranges to respond. In PCI, devices are assigned base addresses during configuration, decoding transactions on the multiplexed address/data bus to access memory or I/O spaces; configuration itself uses a bus-device-function (BDF) addressing scheme accessed via special cycles.[18] In SCSI, addressing occurs during a selection phase: the initiator asserts the target's unique 3- or 5-bit ID (supporting 8 or 16 devices) on the data bus while driving SEL and the target's ID bits, with the target responding by asserting BSY if addressed.[19] Communication in serial multidrop buses generally follows a master-slave model, where a designated master device initiates all transactions by issuing read or write commands, and targeted slave devices respond accordingly. In this setup, the master generates the clock signal and addresses a slave, which then acknowledges and transfers data if the command matches its role.[15] Alternative approaches include polling, where the master sequentially queries each slave for status or data, or token-passing methods, as seen in protocols like ARCNET, where a circulating token grants temporary transmission rights to the holding node, enabling orderly access without a fixed master.[9] Peer-to-peer models, though less common in strict multidrop configurations, permit any node to initiate communication using embedded identifiers, contrasting with the centralized control of master-slave hierarchies.[20] Parallel multidrop buses often support multi-master operation, allowing any device to act as initiator. Transactions proceed in defined phases: an address phase to specify the target and operation, followed by one or more data phases for transfer, controlled by signals like FRAME# in PCI or REQ/ACK handshaking in SCSI. These buses typically operate synchronously with a shared clock signal to coordinate timing across devices.[18][19] Data framing in serial implementations ensures reliable transmission over the shared medium, typically beginning with start and stop bits to delineate messages, supplemented by parity bits for basic error detection in each byte. Packet structures commonly include a header containing the target address and command type, followed by a variable-length payload for data, and concluding with a cyclic redundancy check (CRC) for integrity verification. For example, in Modbus RTU over multidrop RS-485, a frame consists of an 8-bit slave address, an 8-bit function code, the data payload (0-252 bytes), and a 16-bit CRC, allowing the master to poll specific slaves while ignoring irrelevant traffic.[20] This modular format minimizes overhead while enabling slaves to filter messages based on the header address before processing the payload.[20] Parallel buses lack explicit framing, instead using protocol-defined cycles and control signals to structure transactions. Electrically, serial multidrop buses often employ open-drain or open-collector outputs to facilitate multi-device connection without driver conflicts, as these configurations allow any node to pull the line low while the bus idles high. Pull-up resistors connected to the supply voltage ensure the bus returns to a logic-high state when no device is asserting low, with resistor values selected based on bus capacitance and speed requirements—typically 1-10 kΩ for I²C to balance rise times and power consumption.[15] This wired-AND logic supports contention-free addressing phases, where only the addressed slave responds, preventing simultaneous drives from damaging components.[15] In contrast, parallel multidrop buses use tri-state drivers, enabling devices to drive lines actively or enter a high-impedance (Hi-Z) state when idle, allowing safe sharing of address, data, and control lines among multiple devices without conflicts.[21]Collision Detection and Arbitration
In serial multidrop buses, collisions arise when multiple nodes attempt simultaneous transmissions, resulting in overlapping signals that cause interference and data corruption on the shared medium. This interference manifests as distorted waveforms or unexpected bit values, compromising the integrity of the transmitted frames. Detection is achieved by having the transmitting node continuously monitor the bus state during transmission; any discrepancy between the expected output and the actual bus signal indicates a collision, allowing the node to abort promptly.[22] Parallel multidrop buses prevent such collisions through exclusive bus grants, ensuring only one device drives the bus at a time via prior arbitration. Arbitration techniques manage access to resolve conflicts or grant ownership efficiently, differing by topology. In serial buses, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a foundational method where nodes sense the bus for carrier activity before transmitting and, if a collision occurs mid-transmission, terminate the send while propagating a jam signal to alert others. In contrast, non-destructive arbitration, as employed in protocols like the Controller Area Network (CAN), enables concurrent transmissions to proceed without data loss; nodes compare transmitted bits against the bus in a bit-wise manner, with dominant bits (logical 0) overriding recessive ones (logical 1) based on message priority.[23][24] In parallel buses, arbitration is deterministic and collision-free. For PCI, a centralized arbiter connected to all slots uses dedicated REQ# and GNT# lines per device; the arbiter grants bus mastership to one requester at a time, often using round-robin or fixed-priority schemes, with arbitration overlapping the previous transaction to hide latency.[25] In SCSI, distributed arbitration occurs in a dedicated phase: eligible devices release BSY while asserting their ID bits on the data bus (highest ID—most significant bit first—wins), after which the winner proceeds to selection.[19] Resolution protocols vary by system to restore orderly access. In CSMA/CD-based serial setups, colliding nodes implement exponential backoff, delaying retries by a randomized interval that increases with each successive collision to minimize repeated conflicts. Priority-based resolution, such as in CAN, ensures the node with the lowest identifier (highest priority) prevails during arbitration, while lower-priority nodes silently defer without disrupting the winner's frame. Some systems incorporate silent failure modes, where undetected collisions may go unacknowledged, potentially leading to lost messages if monitoring fails.[26] In parallel systems, losers of arbitration simply wait for the next cycle without backoff, relying on the protocol's fairness mechanisms. Error handling emphasizes reliability through recovery mechanisms. Affected nodes retransmit the frame after the arbitration phase concludes, ensuring eventual delivery in non-critical scenarios. For persistent issues, protocols like CAN use error counters to track faults; if a node exceeds thresholds, it enters a bus-off state, temporarily isolating itself from the bus to prevent ongoing disruptions. Addressing mechanisms, which assign unique identifiers prior to transmission, further aid in coordinating access and reducing collision probability.[27]Applications in Computing
Use in Peripheral Interfaces
In personal computers, multidrop buses facilitate the connection of multiple peripherals through shared expansion slots, enabling efficient resource utilization. The Industry Standard Architecture (ISA) bus, a legacy parallel multidrop design from the early 1980s, supported up to 16 slots for cards such as network adapters and storage controllers, allowing simultaneous access via address decoding and interrupt sharing. Similarly, the Peripheral Component Interconnect (PCI) bus, introduced in 1992, operates as a parallel multidrop architecture with up to 10 loads per bus segment, accommodating high-performance peripherals like graphics accelerators and sound cards through centralized arbitration for multiple masters.[28][4] In embedded systems, multidrop buses connect microcontroller peripherals such as sensors and displays using minimal wiring, often just two or three lines, to conserve limited I/O pins on resource-constrained chips. For instance, serial multidrop configurations reduce the pin count by up to 80% compared to dedicated point-to-point links, enabling compact designs in devices like IoT modules and portable gadgets.[29] Performance in these interfaces involves trade-offs due to shared resources: bandwidth is divided among connected devices, potentially limiting throughput to fractions of the bus's maximum capacity, while arbitration mechanisms introduce latency as devices compete for access, often adding several clock cycles per transaction in bus contention scenarios.[7] Representative examples include early printer interfaces that leveraged multidrop expansion buses like ISA for parallel port cards in multi-peripheral setups. Common protocols underpin these interfaces, as detailed in the relevant section.[30]Common Protocols
The Inter-Integrated Circuit (I²C) protocol, developed by Philips Semiconductors (now NXP) in 1982, is a widely adopted multidrop bus standard for short-distance communication between integrated circuits.[15] It employs a two-wire interface consisting of a serial data line (SDA) and a serial clock line (SCL), enabling multi-master and multi-slave configurations where up to 128 devices can be addressed using 7-bit addressing, or more with 10-bit mode.[15] Data rates range from 100 kbit/s in standard mode to 3.4 Mbit/s in high-speed mode, supporting efficient control of peripherals like sensors and EEPROMs in consumer electronics and embedded systems.[15] The protocol's open-drain architecture with pull-up resistors limits bus capacitance to 400 pF, typically constraining effective distances to around 10 meters depending on wiring and loading.[31][32] The System Management Bus (SMBus), introduced by Intel in 1995 as an extension of I²C, targets system management applications in laptops and servers, enhancing reliability for power-sensitive environments.[33] It retains I²C's two-wire structure and addressing but incorporates mandatory features like packet error checking (PEC) using cyclic redundancy checks and bus timeouts (25–35 ms) to prevent hangs and ensure deterministic operation.[33][32] Operating primarily at 10–100 kHz to accommodate low-power devices, SMBus supports protocols such as block transfers and address resolution, making it suitable for monitoring batteries, fans, and temperature sensors without the flexibility of higher I²C speeds.[33][32] Like I²C, its distance is limited by capacitance, generally to short intra-board ranges, though it emphasizes TTL-compatible logic levels (0.8 V low, 2.1 V high) for better noise immunity in managed systems.[32] The 1-Wire protocol, originated by Dallas Semiconductor (now part of Maxim Integrated) in the early 1990s, provides a single-wire bidirectional multidrop interface for low-speed data exchange, particularly in sensor networks.[34] Addressing relies on unique 64-bit ROM identifiers factory-programmed into each device, comprising an 8-bit family code, 48-bit serial number, and CRC checksum, allowing up to billions of devices on a bus without traditional address conflicts.[34] Standard speeds reach 16.3 kbit/s, with an overdrive mode up to 125 kbit/s, and many devices draw parasitic power from the data line, minimizing wiring to one signal plus ground. It excels in applications like temperature sensing and asset tracking, where distances can extend to 100 meters with appropriate bridging, though standard configurations limit to shorter runs for reliability.[35]| Protocol | Max Speed | Typical Distance | Power Characteristics |
|---|---|---|---|
| I²C | 3.4 Mbit/s | ~10 m (capacitance-limited) | Open-drain; moderate consumption via pull-ups (2–10 kΩ) |
| SMBus | 100 kbit/s | Short intra-board (~1–5 m) | Low-power focus; TTL levels for battery systems |
| 1-Wire | 125 kbit/s (overdrive) | Up to 100 m with extensions | Parasitic powering; very low (µA range) |