How does MIL-STD-1553 communication work?

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How does MIL-STD-1553 communication work?

MIL-STD-1553 communication works by designating a single active Bus controller to manage data flow to, from, and between multiple remote terminals. The bus operates in a time-division multiplexed manner, where data is transmitted in messages consisting of “words”.

All words begin with a 3 µs “Sync” field and include a fixed number of data bits (16) and a single parity bit for error-checking each word. There are three types of words: Command words, transmitted by Bus controllers (BCs); Status words, transmitted by remote terminals (RTs); and Data words, transmitted by either BCs or RTs.

Messages are made up of a defined series of words. Different message formats allow the bus controller (BC) to send data to remote terminals (RTs), RTs to send data to the BC, or one RT to send data to another RT.

Broadcast messages let the BC or an RT send data to all RTs on the bus. Meanwhile, “mode code” messages manage bus functions and contain either no data words or just one data word.

MIL-STD-1553 Tutorial and Reference:

Here are some details about how MIL-STD-1553 communication works:

  • Time Division

Only one computer terminal can transmit at any given time, while the other computers listen and receive. This is different from full-duplex systems like Ethernet and RS-232, which allow simultaneous transmit and receive on different wires.

  • Bus Controller

The bus controller is the only device that can transmit command words, which initiate all messages. Command words contain the remote terminal’s address, direction of message transmission, subaddress, and word count.

  • Data Words

The bus controller can send 1 to 32 data words when using BC-to-RT transfers or broadcast data messages. For broadcast messages, all remote terminals accept the data, but no terminals respond with Status words.

  • Remote and Control Devices

The Bus controller, Remote terminals, and Monitor devices are interconnected over two separate buses. Normal operation involves only the primary bus, with the secondary bus available as a redundant backup if the primary bus fails.

  • Message Scheduling

With MIL-STD-1553, most messaging is synchronous; i.e., based on periodic scheduling. In some systems, high-priority aperiodic messages are inserted into gaps between scheduled messages. Whereas, low priority aperiodic messages are inserted at the end of a minor frame.

What is Enhanced Bit Rate 1553?

Known as EBR-1553, this is a communication standard derived from MIL-STD-1553 but engineered for a higher data rate of 10 Mb/s. Instead of operating over a multi-drop bus, it calls for the use of a hub (star) topology and a point-to-point physical layer based on RS-485 transceivers.

What is a MIL-STD-1553 channel?

For MIL-STD-1553, the definition of the word “channel” depends on the context. Sometimes it refers to a dual redundant data bus. Other times, it refers to the fact that a dual redundant bus includes two “channels”. These two channels are sometimes referred to as “Channel A” and Channel B”.

In other contexts, they’re referred to as the “primary channel” and the “secondary channel” or “backup channel”. In either context, the requirements for a MIL-STD-1553 channel include the electrical, mechanical, and operating characteristics of a serial data communication bus defined by the 1553 channel.

What is the purpose of the MIL-STD-1553 bus coupler?

The purpose of a MIL-STD-1553 bus coupler is to maintain signal impedance levels and reduce reflections. Couplers are connected between the MIL-STD-1553 bus and 1553 stubs. Stubs can be up to 20 feet long and connect to Bus controllers, Remote terminals, and Monitors.

They include transformers with a fixed turns stepped-down ratio of 1.4 to 1.0 from the bus to the stub. Additionally, fault-isolation resistors with values of 0.75*Z0 are placed on the bus side of the transformers. These resistors are included to protect the bus from short circuit faults on stubs or in terminals. This prevents a short circuit on a single stub or terminal from shorting out the main bus, which could cause the entire bus to fail.

Couplers also provide enhanced DC isolation, common mode rejection, and lightning protection for connected terminals. This is in addition to the protection offered by the terminals’ internal isolation transformers.

What is the difference between MIL-STD-1553 and ARINC-429?

MIL-STD-1553 and ARINC-429 are both data bus standards that are widely used in avionics systems. ARINC-429 employs a point-to-point(s) physical layer topology and can therefore operate in a single-cast or multicast mode.

ARINC-429 is primarily used in commercial aircraft but is also used in helicopters and other military applications. MIL-STD-1553 operates over a dual redundant multi-drop bus and is ideal for real-time mission-critical applications. It supports single-cast or multicast messages.

Some applications use multi-I/O boards that include both MIL-STD-1553 and ARINC-429 interfaces. Such boards can also include interfaces for other military databus protocols, including EBR-1553 and CAN bus/ARINC-825. Although CANbus is used primarily in commercial automotive applications, it’s also used in military ground vehicles. It is also utilized with heavy industrial and agricultural vehicles.

Is MIL-STD-1553 Ethernet?

No, MIL-STD-1553 is not Ethernet. However, it can be bridged with Ethernet in some systems. MIL-STD-1553 is a military standard for serial data buses that has been used for over 40 years.

While MIL-STD-1553 and Ethernet are different technologies, hybrid systems can combine them by using Ethernet’s OSI model to create two communication channels. This setup can enhance error detection and improve reliability.

What is the difference between MIL-STD-1553 and RS-485?

Like MIL-STD-1553, RS-485 is based on the use of differential signaling. However, in many respects, RS-485 is a less robust standard than 1553. For example, RS-485’s minimum bus voltage is 1.5 volts peak (3.0 volts peak-to-peak), which is half of the MIL- STD-1553 minimum bus voltage of 6.0 volts peak-to-peak. In addition, unlike RS-485, MIL-STD-1553 calls for balanced signaling and transformer isolation.

How fast is the MIL-STD-1553 bus?

MIL-STD-1553 is a serial multiplex data bus with a transmission speed of 1 megabit per second (Mbps). This speed is equivalent to 1 bit per microsecond. The bus is made up of a shielded twisted-wire pair with an impedance of 70 to 85 ohms at 1 MHz.

Transmitter and receiver devices connect to the bus through isolation transformers. In the most commonly used configuration referred to as transformer coupling, there are also transformers built into the couplers. These couplers provide a physical connection to the bus and stubs.

What are the advantages of MIL-STD-1553?

MIL-STD-1553 is reliable due to its physical layer, which offers high noise immunity and reliable data transmission over long distances. Its command/response protocol provides deterministic communication for precise control and validation testing.

Additionally, dual redundancy provides an extra layer of reliability. These features make it a highly dependable data bus for military and aerospace applications.

What is MIL-STD-1553 and how does it work?

MIL-STD-1553 is a message transmission-based bus that defines ten different message types. Most message formats consist of Command words, Status words, and Data words. These words enable communication between system elements.

The bus controller initiates and controls all bus communications, while remote terminal devices attached to the bus respond to the controller’s commands.

The standard defines specifications for terminal devices.

Here are some terms related to MIL-STD-1553:

  • Bus Monitor

CA bus monitor can be configured to receive words from either one or both dual redundant 1553 buses. Bus monitors are passive listeners on 1553 buses. They receive and store words received from the bus but never transmit.

  • Command Word

A type of word transmitted by a bus controller. Its 16 bits include the remote terminal’s 5-bit address, 5-bit subaddress, and a 5-bit word count. There is also a bit to indicate if the remote terminal receiving the Command word will either receive or transmit data.

  • Data Word

All 1553 data words are 16 bits long. Data words may be transmitted by either the BC or an RT. Other than designating the first bit transmitted as the MSB, MIL-STD-1553 does not define the bit definitions and formats of data words. These are typically defined by system-level ICDs (Interface Control Documents).

  • Status Word

Status words are transmitted by Remote Terminals. The first five bits provide the RT’s address and provide acknowledgement to the bus controller. Other status word bits convey the status of the RT or the connected sub-system.

  • Cards

There are multiple types of cards: multi-function, single-function, and dual-function. Multi-function cards can support the three functions simultaneously. This includes managing a bus and emulating one or more RTs at the same time.

Single-function cards can also support the three functions, but not simultaneously. Dual-function cards can support BC + Monitor or RT + Monitor modes simultaneously.

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