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MX Foundation 4
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MX Foundation 4
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Each section below presents a part of the MIL-STD-1553 standard, as described in the Military Standard, Aircraft Internal Time Division, Command/Response Multiplex Data Bus document (21 September 1978).
The word size shall be 16 bits plus 3 bits for the sync waveform and one parity bit for a total of 20 bits. The 16 bits is considered a data, command or status word. The sync pattern is used to distinguish if the 16 bits contains data (data word) or does not contain data (command or status word).
The 20 bits of the command word is composed as follows :
The 20 bits of the data word is composed as follows :
The 20 bits of the status word is composed as follows :
There are two types of messages which can occur on a databus:
The bus controller sends a receive command that specifies the address of the RT which will receive a certain number of words into one of its subaddresses. The command is followed by the indicated number of data words. The command and data words are sent contiguously with no interword gaps. After validating the message, the remote terminal returns a status word to the bus controller. The RT indicates in the status word if the transfer has been successful. If the RT does not receive the specified number of words, it will indicate this in the status word. If the BC does not receive the status in the desired time period, a response time error is set.
The bus controller sends a transmit command to the remote terminal that specifies the address of the RT which transmits a certain number of words from one of its subaddresses. After validating the command word, the remote terminal returns to the bus controller a status word followed by the requested number of data words. The BC waits for the status word up to the response time limit. After this time period, if the status word was not received, the BC detects a response time error. The status and data words are transmitted contiguously with no interword gaps. If the RT does not send the specified number of words, the BC is flag an error.
The bus controller sends a receive command to remote terminal A, immediately followed by a transmit command to remote terminal B. After validating the command, remote terminal B transmits its status word followed by the requested number of data words. The status word and data words are transmitted contiguously with no interword gaps. Once remote terminal B has finished its transmission, remote terminal A transmits its own status word.
The bus controller sends a transmit command to a remote terminal, using one of the previously specified mode codes. To specify a mode command, the subaddress field of the command word shall be set to 00000 or to 11111. The complete list of available mode commands is shown in the "MIL-STD-1553B - DIGITAL TIME DIVISION COMMAND/RESPONSE MULTIPLEX DATA BUS" standard document. After validating the command word, the remote terminal transmits its status word.
The bus controller sends a transmit command to a remote terminal, using one of the previously specified mode codes. To specify a mode command, the subaddress field of the command word shall be set to 00000 or to 11111. The complete list of available mode commands is shown in the "MIL-STD-1553B - DIGITAL TIME DIVISION COMMAND/RESPONSE MULTIPLEX DATA BUS" standard document. After validating the command word, the remote terminal transmits its status word, followed by a single data word. The status and data words are transmitted contiguously with no interword gaps.
The bus controller sends a receive command to a remote terminal, using one of the previously specified mode codes, followed by a single data word. To specify a mode command, the subaddress field of the command word shall be set to 00000 or to 11111. The complete list of available mode commands is shown in the "MIL-STD- 1553B - DIGITAL TIME DIVISION COMMAND/RESPONSE MULTIPLEX DATA BUS" standard document. The command and data words are transmitted contiguously with no interword gaps. After validating the command and data word, the remote terminal returns its status word.
The bus controller sends a receive command with a remote terminal address of 31, followed by the indicated number of data words. The command and data words are transmitted contiguously with no interword gaps. After validating the command and data words, all remote terminals which support the broadcast option sets the broadcast command bit in their status words; however, no status words is actually transmitted.
The bus controller sends a receive command with a remote terminal address of 31, followed by a transmit command to RT A. After validating the command word, RT A transmits its status word followed by the indicated number of data words. The status and data words are transmitted contiguously with no interword gaps. After the transmission, all remote terminals which support the broadcast option (except RT A) will set the broadcast command bit in their status words; however, no status words actually is transmitted.
The bus controller sends a transmit command word with a remote terminal address of 31, and one of the previously specified mode codes. To specify a mode command, the subaddress field of the command word shall be set to 00000 or to 11111. The complete list of available mode commands is shown in the "MIL-STD-1553B - DIGITAL TIME DIVISION COMMAND/RESPONSE MULTIPLEX DATA BUS" standard document. After validating the command word, the remote terminals which support the broadcast option will set the broadcast command bit in their status words; however, no status words will actually be transmitted.
The bus controller sends a receive command word with a remote terminal address of 31, and one of the previously specified mode codes, followed by a single data word. To specify a mode command, the subaddress field of the command word shall be set to 00000 or to 11111. The complete list of available mode commands is shown in the "MIL-STD-1553B - DIGITAL TIME DIVISION COMMAND/RESPONSE MULTIPLEX DATA BUS" standard document. The command and data word are transmitted contiguously with no interword gaps. After validating the command word, the remote terminals which support the broadcast option will set the broadcast command bit in their status words; however, no status words is actually transmitted.
The bus controller waits a minimum of 4 microseconds after the end of one databus transaction before transmitting a new command. This gap is measured from the mid bit zero crossing of the previous transaction’s parity to the mid-zero crossing of the next command word sync.
A remote terminal shall respond to a valid command word within the time period of 4.0 to 12.0 microseconds. This time period is measured from the mid bit zero crossing of the last transmitted parity to the mid-zero crossing of the status word sync.
The MIL-STD-1553 standard defines three different terminal modes:
A terminal operating as a bus monitor shall receive bus traffic and extract selected information. While operating as a bus monitor, the terminal shall not respond to any messages. All information obtained while acting as a bus monitor shall be strictly used for off-line applications (e.g., flight test recording, maintenance recording or mission analysis) or to provide the back-up bus controller sufficient information to take over as the bus controller.
A terminal operating as a bus controller shall be responsible to send data bus commands, participating in data transfers, receiving status responses, and monitoring system status as defined in the MIL-STD-1553 standard. The bus controller function may be embodied as either a stand-alone terminal, whose sole function is to control the data bus(s), or contained within a subsystem. Only one terminal shall be in active control of a data bus at any one time.
A remote terminal (RT) shall operate in response to valid commands received from the bus controller. The RT shall accept a command word as valid when the command word meets the criteria of 4.4.1.1 of MIL-STD-1553, and the command word contains a terminal address which matches the RT address or an address of 11111, if the RT has the broadcast option. No combination of RT address bits, T/R bit, subaddress/mode bits, and data word count/mode code bits of a command word shall result in invalid transmissions by the RT. Subsequent valid commands shall be properly responded to by the RT.
The standard also defines the RT as “All terminals not operating as the bus controller or as a bus monitor.” RTs are basically used to transfer messages from the main data bus to a subsystem. A subsystem is defined within the standard as “the device or functional unit receiving data transfer service from the data bus".
With FX1553-4, FM1553-4, FMX-M1553-4 and IPM-1553-MRT (MAX-IP-2162-AEK, MAX-IP-2162-2AEK, MAX-IP-2166-AEK and MAX-IP-2166-2AEK) devices, each TX subaddress can be configured to send a checksum in place of the last data word of the message. Refer to the mxfMIL1553MsgChecksumEnableSet() function.
