The sixth bit is 0 for Receive or 1 for Transmit. The next 5 bits indicate the location (sub-address) to hold or get data on the Terminal (1–30). Note that sub-addresses 0 and 31 are reserved for Mode Codes. The last 5 bits indicate the number of words to expect (1–32). All zero bits indicate 32 words. In the case of a Mode Code, these bits indicate the Mode Code number (e.g., Initiate Self Test and Transmit BIT Word). The Status Word decodes as follows. The first 5 bits are the address of the Remote Terminal that is responding. The rest of the word is single bit condition codes, with some bits reserved. A 'one' state indicates condition is true. More than one condition may be true at the same time. The image below exemplifies many of the protocol and physical layer concepts explained above. For example, the RT address contained in the Command Word has a value of 0x3 (in range of 0 to 31). The sixth bit is 1, indicating a Transmit from the RT. The sub-address is 0x01. The last 5 bits indicate the number of words to expect take a value of 1, which is matched by the single Data Word (value 0x2) after the Status Word. Also as explained above, devices have to start transmitting their response to a valid command within 4–12 microseconds. In the example, the Response Time is 8.97 us, therefore within specifications. This means that the Remote Terminal (RT) number 3 has responded to the Bus Controller query after 8.97 us. The amplitude of the query is lower than the amplitude of the response because the signal is probed at a location closer to the Remote Terminal. In the Status Word, the first 5 bits are the address of the Remote Terminal that is responding, in this case 0x3. A correct Transfer exhibits the same RT address in the Command Word as in the Status Word. Conceptual description Figure 1 shows a sample MIL-STD-1553B system that consists of: redundant MIL-STD-1553B buses a Bus Controller a Backup Bus Controller a Bus Monitor a standalone Remote Terminal with one or more subsystems communicating with it a subsystem with an embedded Remote Terminal The Bus Controller There is only one Bus Controller at a time on any MIL-STD-1553 bus. It initiates all message communication over the bus. Figure 1 shows 1553 data bus details: