HI-6121PQMF HOLTIC [Holt Integrated Circuits], HI-6121PQMF Datasheet - Page 52

• Manufacturer Part Number: HI-6121PQMF
• Description: MIL-STD-1553 Remote Terminal ICs
• Manufacturer: HOLTIC [Holt Integrated Circuits]

MESSAGE DATA BUFFERS, Cont.
9
8
7-6
5
PING-PONG DATA BUFFERING
DOUBLE-BUFFERED (PING-PONG) MODE
Ping-pong buffer mode is a method for storing message
and time-tag information and data associated with
messages. Each unique MIL-STD-1553 subaddress and
mode code is assigned a pair of data buffers for transmit
commands and a pair of data buffers for receive
commands. The device retrieves buffer data for transmit
commands, or stores buffer data for receive commands.
During ping-pong operation, the device alternates
message storage between Data Buffer A and Data Buffer
B, on a message-by-message basis.
When a subaddress or mode command uses ping-pong
data buffer mode, its 4-word descriptor block in the
Descriptor Table is defined as follows:
If Descriptor Word 1 is stored at memory address N,
Descriptor Word 2 is stored at address N+1, and the other
two words are stored at addresses N+2 and N+3.
Prior to starting terminal operation, enable ping-pong
buffering for any subaddress (or mode code) by asserting
the PPEN bit and negating the STOPP bit in the descriptor
Control Word. When the device detects ping-pong is
selected (PPEN = 1) and enabled (STOPP = 0), it asserts
the Control Word PPON bit to confirm ping-pong is active.
During ping-pong operation, the RT determines the active
data buffer at the beginning of message processing. The
Control Word DPB bit indicates the data pointer to be used
4-0
Descriptor Word 1
Descriptor Word 2
Descriptor Word 3
Descriptor Word 4
WASBSY
ILCMD
——
BUSID
MC4:0
Was Busy Status.
This bit is asserted when the terminal responds to the mode command with BUSY status, due to
global BUSY bit set in 1553 Status Bits Register, or command-specific MKBUSY bit set in the
descriptor table Control Word. No mode data word was transmitted.
Illegal Command Received.
This bit is asserted when the Illegalization Table bit corresponding to the received command is logic
1. The Illegalization Table should only contain nonzero values when “illegal command detection” is
being applied. See section entitled Illegalization Table for further information.
Not used.
Bus Identification.
If this bit equals zero, message was transacted on Bus A. If bit equals one, it was transacted on Bus B.
Mode Code.
This 5-bit field contains the mode code extracted from the command word.
Control Word
Data Pointer A
Data Pointer B
Broadcast Data Pointer
HOLT INTEGRATED CIRCUITS
HI-6120, HI-6121
52
by the next command. DPB equals logic 0 means Data
Pointer A is used next; DPB equals logic 1 means Data
Pointer B is used next. For ping-pong, Data Pointers A and
B are static values pointing to the first address in each
buffer. At the conclusion of error-free message processing,
the Control Word DPB bit is inverted so the next command
“ping-pongs” to the other data buffer. Each new message
to the subaddress or mode code overwrites message data
and information words written two messages back. The
DPB bit does not toggle when a message ends in error, or if
the command was illegal, or if Busy status applied for the
received command. In these cases, the next command will
overwrite the same buffer. Figure 11 is a general illustration
of ping-pong buffer mode. Figure 12 shows a specific
example.
PING-PONG ENABLE / DISABLE HANDSHAKE
Because ping-pong messages and host buffer servicing
are asynchronous, there is potential for “data collision”.
Here is a data collision example: The host reads data from
an earlier message while the device simultaneously writes
new message data to the same buffer. The host reads a
mix of new and old message data. Collisions can occur for
both transmit and receive messages.
A handshake scheme lets the external host
asynchronously service ping-pong data buffers without
data collision. To off-load or load a subaddress (or mode
code) buffer, the application software performs the
following sequence:
(a) Host asserts the Control Word STOPP bit to suspend
ping-pong operation for the subaddress. When the device
recognizes STOPP bit assertion, it negates the PPON bit