MIL-STD-1553B Mode Code Reference
Look up 1553B mode codes by T/R bit and code, with data-word rules.
| Code | 5-bit | T/R | Data word | Function |
|---|---|---|---|---|
| 0 | 00000 | 1 (Tx) | no | Dynamic Bus Control |
| 1 | 00001 | 1 (Tx) | no | Synchronize |
| 2 | 00010 | 1 (Tx) | no | Transmit Status Word |
| 3 | 00011 | 1 (Tx) | no | Initiate Self-Test |
| 4 | 00100 | 1 (Tx) | no | Transmitter Shutdown |
| 5 | 00101 | 1 (Tx) | no | Override Transmitter Shutdown |
| 6 | 00110 | 1 (Tx) | no | Inhibit Terminal Flag Bit |
| 7 | 00111 | 1 (Tx) | no | Override Inhibit Terminal Flag Bit |
| 8 | 01000 | 1 (Tx) | no | Reset Remote Terminal |
| 16 | 10000 | 1 (Tx) | yes | Transmit Vector Word |
| 17 | 10001 | 0 (Rx) | yes | Synchronize (with data word) |
| 18 | 10010 | 1 (Tx) | yes | Transmit Last Command Word |
| 19 | 10011 | 1 (Tx) | yes | Transmit Built-In-Test (BIT) Word |
| 20 | 10100 | 0 (Rx) | yes | Selected Transmitter Shutdown |
| 21 | 10101 | 0 (Rx) | yes | Override Selected Transmitter Shutdown |
Codes 9–15 and 22–31 are reserved. Mode commands use subaddress 0 or 31.
This is a searchable reference for MIL-STD-1553B mode codes — the command set that manages the bus rather than moving data. Filter by code or name to find the T/R bit, whether the command carries a data word, and its function. Mode commands are signalled by subaddress 0 or 31; the five word-count bits then hold the mode code.
How it works
When a command word's subaddress is 0 or 31, the terminal reads the last five bits as a mode code instead of a word count. The standard divides these into two groups: codes 0-15 command an action with no accompanying data word (Transmit Status Word, Reset Remote Terminal, Initiate Self-Test, and so on), while codes 16-21 are paired with a single data word and are further distinguished by the T/R bit. Reserved codes fill the gaps.
The defined codes at a glance
Without a data word (codes 0–8, all T/R = 1): Dynamic Bus Control (0), Synchronize (1), Transmit Status Word (2), Initiate Self-Test (3), Transmitter Shutdown (4), Override Transmitter Shutdown (5), Inhibit Terminal Flag Bit (6), Override Inhibit Terminal Flag Bit (7) and Reset Remote Terminal (8).
With a data word (codes 16–21): Transmit Vector Word (16, T/R 1), Synchronize with data word (17, T/R 0), Transmit Last Command Word (18, T/R 1), Transmit Built-In-Test Word (19, T/R 1), Selected Transmitter Shutdown (20, T/R 0) and its override (21, T/R 0). Everything else — 9–15 and 22–31 — is reserved.
Worked examples
mode code 2, T/R 1 → Transmit Status Word (no data word)
mode code 17, T/R 0 → Synchronize (with data word) — one data word follows
Parameters
| Parameter | Value | Notes |
|---|---|---|
| Trigger | subaddress 0 or 31 | |
| Codes 0–8 | T/R 1 · no data word | bus / terminal management |
| Codes 16–21 | with data word | T/R selects direction |
| Reserved | 9–15, 22–31 |
FAQ
▸When does a 1553 command carry a mode code?
When the subaddress field is 0 or 31. That signals a mode command, so the last five bits of the command word are a mode code (0-31) rather than a word count. Mode commands manage the bus and terminals rather than moving payload data.
▸How does the T/R bit affect a mode code?
It selects which mode command a code means. Codes 0-15 are defined for T/R = 1 (transmit) and carry no data word. Codes 16-21 are associated with a data word and split by direction — for example code 17 (Synchronize with data word) uses T/R = 0, while code 16 (Transmit Vector Word) uses T/R = 1.
▸Which mode codes have a data word?
Codes with bit 4 set (16 and above): Transmit Vector Word (16), Synchronize with data word (17), Transmit Last Command (18), Transmit BIT Word (19), Selected Transmitter Shutdown (20) and its override (21). Codes 0-15 have no data word.
▸What are the reserved codes?
Codes 9-15 (no data word) and 22-31 (with data word) are reserved by MIL-STD-1553B and should not be used for custom functions. Encountering them in a capture usually points to a bit error or a non-conformant terminal.
▸Which mode codes use T/R = 0 (receive)?
Three of the data-word codes: Synchronize with data word (17), Selected Transmitter Shutdown (20) and Override Selected Transmitter Shutdown (21). In each case the bus controller sends a data word to the terminal, so the command is a receive from the RT's point of view. The other defined codes are transmit commands (T/R = 1).
▸Is subaddress 0 different from subaddress 31?
No — the standard designates both as mode-command indicators, and a terminal must decode either one the same way. Which of the two a given system uses is a program convention, so a capture may show either.
▸How do I decode the rest of the command word?
A 1553 command word is 16 bits: RT address (bits 15-11), T/R bit (bit 10), subaddress (bits 9-5) and word count or mode code (bits 4-0). The Command Word tool on this site breaks a hex word into those fields, and the Message Decoder handles full message sequences.
▸Is this the official assignment?
The table reflects the mode code assignments in the public MIL-STD-1553B standard. Always confirm against the controlling document for your program; some legacy systems restrict or extend the set.
Related tools
MIL-STD-1553B Command Word Decoder
Decode and build 1553B command words: RT address, T/R, subaddress, word count, mode codes.
MIL-STD-1553B Status Word Decoder
Decode 1553B status words: RT address and every status flag bit.
MIL-STD-1553B Message Decoder
Lay out a full 1553B transaction: command, data words and status, with parity checks.