MIL-STD-1553B Avionic Data Bus

Introduction:

Originally an avionics data bus for use by the United States military, MIL-STD-1553 was first adopted in 1973 by the US Air Force and first used on the F-16 fighter jets. Subsequent revisions later (the latest at time of writing being revision C in 2018), and the data bus has been adopted in civil aerospace application, and in spacecraft, including on the James Webb space telescope and International Space Station.

It is now maintained by both the U.S. Department of Defence and the Society of Automotive Engineers. Outside of the US, and adopted by NATO, MIL-STD-1553 has been re-packaged with tweaks as STANAG 3838 and by the UK MoD as Def-Stan 00-18 part 2 where it has been used together with STANAG 3910 on the Eurofighter Typhoon.

Topology:

A MIL-STD-1553 network begins with a Bus Controller (BC) connected to wire pair of 70 Ω to 85 Ω impedance at 1 MHz. A shielded twisted pair, twinaxial, coaxial, and triaxial cable can all be used. The Bus Controller initiates all communications, sending commands at predetermined intervals.

 

To the bus cable up to 31 compatible devices can be attached via coupling ‘stubs’, and isolation transformers either external or built into the device. The first of the devices we will connect are Remote Terminals (RT). Each Remote Terminal is assigned a unique 5 bit address, ranging from 0 to 30. Address 31 (binary 11111) is reserved for a special purpose, a broadcast mode which we shall come to later. The Remote Terminal receives and decodes the commands from the Bus Controller, and responds accordingly.

 

A Remote Terminal might be either an interface between the MIL-STD-1553 and a subsystem, or built into the subsystem itself, for example an Air Data Inertial Reference Unit (ADIRU), a switch on the landing gear or lighting. It may also be a device bridging two MIL-STD-1553 buses.

Some installation will also include a Bus Monitor (BM) to passively record the traffic for later analysis. It may record all or a filtered subset of the bus messages including errors.

 

The resilience of the bus is enhanced by the addition of a second wire pair to which the Bus Controller and Remote Terminals are be coupled. A fallback or backup Bus Controller is also be installed. N.B. Only one bus controller and one bus wire pair can be active at any one time.

 

The bus cables must be terminated with a resistance equal to the cable’s nominal impedance, typically 78Ω, this helps to prevent disruption and intermittent communication failures.

Bit Representation and Bus Protocol:

Messages on a MIL-STD-1553 bus comprise of one or more 16 bit words. Words come in three types; Command, Data, and Status. Each bit is represented by a 1 µs Manchester Coded bi-phase pulse which conveys both data and clock. To aid synchronisation, each word is preceded by a 3 µs sync pulse and followed by an odd parity bit. Thus a word could be seen as 20 bits long. A longer 4 µs gap is introduced between messages with the expectation that all devices will broadcast a reply to a command within 4 to 12 µs. If after 14 µs a device has not responded, it is judged not to have received the command.

The Command Word:

Each MIL-STD-1553 command word is a 16-bit value that contains four fields; Remote Terminal Address, Transmit / Receive, Subaddress / Mode, and Data Word Count / Mode Code. This 16-bit value is sandwiched between a 3-bit long sync pulse at the front and a single parity bit at the end.

The diagram below shows the first transmitted bit on the left, the last on the right.

Command Word Format
Sync.			Remote Terminal Address					T/R	Subaddress / Mode					Data Word Count / Mode Code					Parity
1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20

 

We can rotate this table through 90° and expand it to include a full description.

Command Word Format, Expanded
Section	Bit Count	Description
Sync	1	To aid synchronisation, each word is preceded by a 3 µs sync pulse. The sync pulse is positive for the first half and negative for the second half. The sync pulse is invalid as a Manchester Code.
	2
	3
Source Remote Terminal Address	4	This is the 5 bit address of the Remote Terminal designated to action the command. All but binary address 11111 (decimal 31) identify a unique Remote Terminal. Instead decimal 31 addresses all devices on a “Broadcast Mode”, if supported by the bus hardware.
	5
	6
	7
	8
Transmit / Receive	9	Transmit / Receive Value Interpretation 0 The RT is to receive data following the command. 1 The RT is to transmit data following the command.
Subaddress / Mode	10	Subaddress / Mode Value Interpretation 00000 The Data Word Count should be decoded as Mode Commands. 00001 A subaddress that identifies a specific function of the RT, selecting particular data set to be transmitted, or written. ..... 11110 11111 The Data Word Count should be decoded as Mode Commands.
	11
	12
	13
	14
Data Word Count / Mode Code	15	The Subaddress / Mode value (above) determines how these 5 bits are to be read, this will be either: a) The number of data words to be transmitted or received, where binary 00001 through to 11111 are 1 to 31 decimal, and binary 00000 is actually 32. b) The Optional Mode Code, expanded below.
	16
	17
	18
	19
Parity	20	Used for error detection, this bit is used to ensure that the total number of 1 bits in the previous 16 bits is an odd number. For example the command word 10100 1 00101 01010 has 7 1 bits. This is already an odd number, so the parity bit will be set to 0.

The Optional Mode Control Code:

When multiplexed bus related hardware is used, and to assist with information flow management, the Data Word Count value on the Command Word may contain an Mode Code instead. As we shown above, this is determined by using a value of 00000 or 11111 in the Subaddress / Mode section.

The use of an Mode Code also affects the interpretation of the transmit / receive bit as described below.

Mode Codes 00000 to 01111 reference modes that do not require the transfer of a data word. In these cases the T/R bit should be set to 1.

Mode Codes 10000 to 11111 reference modes that expect the transfer of a single 16 bit data word. In these cases the T/R bit indicates the direction of the transfer along the lines prescribed above.

Here’s a table of the Mode Codes and their functionality.

MIL-STD-1553 Mode Control Code:
T/R	Mode Control	Function	Description	Data Word	Broadcast
1	00000	Dynamic Bus Control	Hands control of the bus to a RT. If the RT declines, the existing bus controller retains control.	No	No
1	00001	Sync.	Reset the internal timer of an RT, initiate a sequence, etc..	No	Yes
1	00010	Transmit Status Word	Causes an RT to transmit the status word associated with the previous valid command so that the BC can confirm receipt.	No	No
1	00011	Initiate Self-Test	Causes an RT to begin a self-test and the transmit it’s status.	No	Yes
1	00100	TX Shutdown	Disables the TXassociated with the current redundant / backup bus.	No	Yes
1	00101	Override TX	Enables the TX associated with a previously disabled redundant bus.	No	Yes
1	00110	Inhibit Terminal Flag Bit	Sets the T/F bit in the status word to logical zero unless otherwise commanded.	No	Yes
1	00111	Override Inhibit Terminal Flag Bit	Resets the Inhibit Terminal Flag bit.	No	Yes
1	01000	Reset Remote Terminal	Resets an RT to the default power up initialised state.	No	Yes
1	01001	Reserved	Not currently used.	No	TBD
1	.....	Reserved	Not currently used.	No	TBD
1	01111	Reserved	Not currently used.	No	TBD
1	10000	Transmit Vector Word	Cause the RT to transmit a status word and a data word containing service request information.	Yes	No
0	10001	Sync.	The RT shall receive a Command Word followed by a Data Word containing sync. information for the RT.	Yes	Yes
1	10010	Transmit Last Command	Causes an RT to transmit its Status Word.	Yes	No
1	10011	Transmit Built-In-Test Word	Causes an RT to transmit its Status Word followed by a single Data Word containing the Built-In-Test (BIT) data.	Yes	No
0	10100	Selected TX Shutdown	Causes an RT to disable the TX associated with a specific redundant bus.	Yes	Yes
0	10101	Override Selected TX	Enables a TX previously disables by the Selected TX Shutdown command.	Yes	Yes
0 or 1	10110	Reserved	Not currently used.	Yes	TBD
0 or 1	.....	Reserved	Not currently used.	Yes	TBD
0 or 1	11111	Reserved	Not currently used.	Yes	TBD

where TX = Transmitter

The Data Word:

Each MIL-STD-1553 data word is a 16-bit value sandwiched between a 3-bit long sync pulse at the front and a single parity bit at the end. These contain the information being sent to or from a Bus Controller or Remote Terminal. A maximum of 32 Data Words may follow a Command Word.

The diagram below shows the first transmitted bit on the left, the last on the right.

Data Word Format
Sync.			Data																Parity
1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20

The Status Word:

Each MIL-STD-1553 status word is a 16-bit value containing ten values / flags, sandwiched between a 3-bit long sync pulse at the front and a single parity bit at the end.

The diagram below shows the first transmitted bit on the left, the last on the right.

Status Word Format
Sync.			Remote Terminal Address					ME	I	SR	Reserved			BC	B	SF	DBC	TF	Parity
1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20

 

We can rotate this table through 90° and expand it to include a full description.

Status Word Format, Expanded
Section	Bit Co­unt	Description
Sync	1	To aid synchronisation, each word is preceded by a 3 µs sync pulse. The sync pulse is positive for the first half and negative for the second half. The sync pulse is invalid as a Manchester Code.
	2
	3
Source Remote Terminal Address	4	This is the 5 bit address of the Remote Terminal transmitting the Status Word. All but binary address 11111 (decimal 31) identify a unique Remote Terminal. Decimal 31 addresses all devices on a “Broadcast Mode”, if supported by the bus hardware.
	5
	6
	7
	8
Message Error	9	Message Error Bit Value Interpretation 0 No error. 1 One of the following error conditions may have occurred. A received Data Word contains an error. Incorrect number of Data Words received. Unrecognised Command Word received.
Instru­men­tation	10	Used to aid the distinction between Command and Status Word. Usually set to logical zero.
Service Request	11	Optional bit used to request the Bus Controller to carry out a predefined action.
Reserved	12
	13
	14
Broadcast Command	15	Broadcast Command Bit Value Interpretation 0 Previous command was not a broadcast command. 1 Previous command was a broadcast command.
Busy	16	Busy Bit Value Interpretation 0 Not busy, or not implemented. 1
Subsystem Flag	17	Subsystem Flag Bit Value Interpretation 0 No error, or not implemented. 1 There is a fault on the RT’s subsystem.
Dynamic Bus Control Acceptance	18	Dynamic Bus Control Acceptance Bit Value Interpretation 0 RT has rejected hand over of bus control. 1 RT has accepted hand over of bus control.
Terminal Flag	19	Terminal Flag Bit Value Interpretation 0 No fault, or not implemented. 1 RT has a fault.
Parity	20	Used for error detection, this bit is used to ensure that the total number of 1 bits in the previous 16 bits is an odd number. For example the command word 10100 1 00101 01010 has 7 1 bits. This is already an odd number, so the parity bit will be set to 0.

Message Formats:

Ten types of message format are defined by MIL-STD-1553B.

The first six are information transfer formats and are shown below where ...

#	Indicates an inter-message gap.
**	Indicates the response time.
RX	Receive.
TX	Transmit.

 

Bus Controller to Remote Terminal Transfers
RX Co­m­m­and	Data Word	Data Word	•••••	Data Word	٭  ٭	Status Word	#	Next Co­m­m­and
Remote Terminal to Bus Controller Transfers
TX Co­m­m­and	٭  ٭	Status Word	Data Word	Data Word	•••••	Data Word	#	Next Co­m­m­and
Remote Terminal to Remote Terminal Transfers
RX Co­m­m­and	TX Co­m­m­and	٭  ٭	Status Word	Data Word	•••••	Data Word	٭  ٭	Status Word	#	Next Co­m­m­and
Mode Command without Data Word
Mode Co­m­m­and	٭  ٭	Status Word	#	Next Co­m­m­and
Mode Command with Data Word (TX)
Mode Co­m­m­and	٭  ٭	Status Word	Data Word	#	Next Co­m­m­and
Mode Command with Data Word (RX)
Mode Co­m­m­and	Data Word	٭  ٭	Status Word	#	Next Co­m­m­and
Four message types are broadcast information formats.
Bus Controller to Remote Terminal Broadcast
RX Co­m­m­and	Data Word	Data Word	٭  ٭	Data Word	#	Next Co­m­m­and
Remote Terminal to Remote Terminal Broadcast
RX Co­m­m­and	TX Co­m­m­and	٭  ٭	Status Word	Data Word	•••••	Data Word	#	Next Co­m­m­and
Mode Command without Data Word
Mode Co­m­m­and	#	Next Co­m­m­and
Mode Command with Data Word (RX)
Mode Co­m­m­and	Data Word	#	Next Co­m­m­and

Disclaimer: The information in this guide is designed to help you understand our products. It is correct to the best of our knowledge and at the time of writing. However Photo-Sonics International Limited does not accept any liability for its use. Instead the latest documentation from the relevant authority should be consulted.