HI-6120_12 Datasheet, PDF (46/158 Pages) Holt Integrated Circuits – MIL-STD-1553 Remote Terminal ICs

HI-6120, HI-6121
6. COMMAND RESPONSES
A brief review of MIL-STD-1553 commands and re-
sponses is appropriate here to establish terminology
used in the rest of this data sheet. Shown in Figure 3,
each command word is comprised of a sync field, three
5-bit data fields, a single bit denoting Transmit / Receive
direction and ends with a parity bit. The hardware de-
coder uses the sync field to determine word type (com-
mand vs. data). Word validity is based on proper sync
encoding, Manchester II encoding, correct bit count and
correct odd parity for the 16 data bits. Once a valid word
with command sync is found, the sync and parity are
stripped before the command’s 16 data bits are stored
for further processing.
Command
T/R
Sync
Bit
Word Count
Field *
Terminal
Address
Field
Subaddress
(SA) Field
* Word Count field is replaced by Mode Code
field when the SA field equals 0x00 or 0x1F
Parity
Bit
Figure 3.  MIL-STD-1553 Command Word
Structure
A “valid command” can be specifically addressed to the
individual HI-6120 terminal (the command word’s em-
bedded Terminal Address field matches the terminal
address latched in the Operational Status register) or
can be a “broadcast command” addressed to all termi-
nals. Broadcast commands are always addressed to RT
address 31 (0x1F). In systems where broadcast com-
mands are disallowed, RT31 is not used as a conven-
tional terminal address. When set, the BCSTINV bit in
Configuration Register 1 renders RT31 commands as
“invalid”: broadcast commands are indistinguishable
from commands addressed to other terminals. Invalid
commands are simply disregarded.
When the command word’s 5-bit SA (subaddress) field
is in the range of 1 to 30 (0x01 to 0x1E) the command is
considered a “subaddress command”. The terminal will
either receive or transmit data words, and “direction” is
specified by the command’s T/R bit. The number of data
words transacted is specified in the 5-bit word count
field, ranging from 1 to 32 words. Thirty-two data words
is represented when the word count field equals 0x00.
When the command’s 5-bit subaddress field equals 0 or
31 (0x1F) a “mode code” command is indicated; the low
order five bits no longer specify a word count, instead
they convey a mode code value. This data sheet refers
to mode code commands by the mode code number. For
example, a mode command with 5-bit mode code field
of 0x10 is called MC16, and the full range of mode code
values is MC0 through MC31 (decimal).
Mode codes MC16 through MC31 (0x10 through 0x1F)
have a single associated data word. When the command
T/R bit equals 0, the data word is contiguous with the
command word and received by the RT. When the com-
mand’s T/R bit equals 1, the data word is transmitted by
the RT,following the terminal’s transmitted status word.
Mode codes MC0 through MC15 (0x0F) do not have as-
sociated data words. For these 16 commands, the com-
mand T/R bit does not specify “direction”. These com-
mands must be transmitted with T/R bit equal to 1. If the
T/R bit is 0, the mode command is “undefined”.
Twenty-two mode commands are “undefined mode com-
mands ” in MIL-STD-1553B:
Mode Codes 0 through 15 with T/R bit = 0
Mode Codes16, 18 and19 with T/R bit = 0
Mode Codes 17, 20 and 21 with T/R bit = 1
The UMCINV bit in Configuration Register 1 determines
how these undefined mode commands are handled
by the HI-6120/21. If the UMCINV configuration bit
equals 1, the undefined mode commands are treated as
invalid. They are not recognized by the device. There is
no terminal response and status is not updated. If the
UMCINV configuration bit equals 0, the 22 undefined
mode commands are considered valid; this is the
default condition following reset. For this case, terminal
response depends on whether or not the application
uses “illegal command detection.”
If illegal command detection is not used, all Illegaliza-
tion Table entries should be logic 0, including the 22 en-
tries for these undefined commands. (The Illegalization
Table is fully described in Section 7 on page 48. After MR
reset, all entries equal logic 0.) The terminal responds
“in form”, transmitting clear status (and a single mode
data word if the command is MC17, MC20 or MC21 with
T/R bit = 1). Terminal status is updated.
If illegal command detection applies, the Illegalization
Table entries for these 22 undefined commands should
be initialized to logic 1. In this case, the terminal will re-
spond with status word only, with Message Error bit set.
No mode data word is transmitted. Terminal status is up-
dated.
Twenty-seven mode codes are considered “reserved” in
MIL-STD-1553B:
HOLT INTEGRATED CIRCUITS
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