Command word
RT (15–11) · T/R (10) · subaddress (9–5) · word count (4–0)
T/R = 1: the terminal transmits; 0: it receives. A word count of 0 means 32 words, the most a message carries.
// DECODER
MIL-STD-1553 word decoder.
Type the 16 bits of a command, status or data word in hexadecimal, as a bus monitor or an interface card shows them. The decoder names the remote terminal, the direction, the subaddress and the word count, recognises mode codes and the broadcast rules, and lays out the flags of a status word.
Runs in your browser. Nothing is sent.
RESULTS
Remote terminal
RT 5
Message
Transmit 4 words from subaddress 3
Terminal to bus controller
The word, first bit sent on the left
0010110001100100 RT 5 T/R 1 Subaddr. 3 Count 4 RT 5 ME 1 INS 0 SR 0 Res. 011 BCR 0 BSY 0 SF 1 DBA 0 TF 0 Data 2C64
| Word | |
|---|---|
| Hexadecimal | 2C64 |
| Parity bit on the bus | 1 (odd parity over the 16 bits) |
| Command word | |
| T/R (bit 10) | 1: the terminal transmits |
| Subaddress (bits 9–5) | 3 |
| Word count or mode code (bits 4–0) | 4 words |
| Data words in the message | 4, after the status word |
| Broadcast | No |
| Status word | |
| Message error (bit 10) | |
| Instrumentation (bit 9) | |
| Service request (bit 8) | |
| Reserved (bits 7–5) | |
| Broadcast command received (bit 4) | |
| Busy (bit 3) | |
| Subsystem flag (bit 2) | |
| Dynamic bus control acceptance (bit 1) | |
| Terminal flag (bit 0) | |
| Data word | |
| Binary | |
| Bytes | |
Check the highlighted field: every box needs a valid number.
HOW IT IS COMPUTED
Twenty bit times, three kinds of word.
On the bus each word lasts 20 µs: a sync pattern of three bit times, the 16 bits decoded here, then an odd parity bit. Interface cards hand over the 16 bits, bit 15 being the first sent.
Mode commands
subaddress 0 or 31 → bits 4–0 = mode code
Codes 0 to 15 carry no data word; codes 16 to 31 carry one, sent by the terminal when T/R = 1 and to it when T/R = 0. The decoder names the codes MIL-STD-1553B defines and flags the reserved and undefined ones.
Broadcast
RT 31 → every terminal, no status word in reply
Where a system uses broadcast, address 31 reaches every terminal at once. A transmit command cannot be broadcast, since no terminal could answer alone, and only some mode codes may be.
Status word
RT (15–11) · ME · INS · SR · reserved (7–5) · BCR · busy · SF · DBCA · TF
The terminal's reply. The instrumentation bit is always 0 in a status word; systems that use the option set the same bit, the first of the subaddress, to 1 in their command words. A 1 there points to a command word.
Command or status?
same sync, same first five bits
Both start with the command/status sync and a terminal address: only their place in the message tells them apart. That is why the kind of word is yours to choose. Data words have a sync of their own.
Field names follow MIL-STD-1553B and its Notice 2. What each subaddress carries is defined by the interface control document of each terminal. The example is a transmit command to terminal 5, subaddress 3, for 4 words.
Further reading
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