OUTPUT FLAGS
The CT1775 provides numerous output flags to offer the
greatest user flexibility. VALID WORD indicates receipt of a
word with valid sync, Manchester coding and parity. RT
ENABLE indicates a valid word and correct address. VAL
CMD WD indicates a valid word and a Command Sync.
BROADCAST indicates a valid command word and an address
of 11111. The BROADCAST flag may be inhibited by using
the BDCST INHIBIT line. MODE CODE indicates a valid
command word and a subaddress of 11111 or 00000.
For multiple word transmissions, the next word may be
transferred to the transmit register any time after SEND DATA
goes HIGH, but no later than the next LOW to HIGH transition
of SEND DATA. SEND DATA remains HIGH for 16 periods
of ESC OUT, during which time the data word is serially
shifted to the Manchester encoder. The encoder adds the parity
bit during the next ESC OUT period after SEND DATA goes
LOW. To terminate transmission after any word, ENC
ENABLE must go to HIGH no later than the first rising edge of
ESC OUT after SEND DATA goes LOW.
The entire transmit cycle may be interrupted and initialized by
applying a 1 microsecond (minimum) negative pulse to MRST.
It is possible to input data to the encoder in serial form by
forcing both transmit registers to be transparent. With LATCH
DATA 1 held HIGH and LOAD DATA 1 held LOW, serial data
input on D15 will be applied directly to the encoder serial
input. ESC OUT must be used to shift in the serial data, MSB
first, starting at the LOW to HIGH transition of SEND DATA.
INITIALIZATION
To ensure error-free operation, it is desirable to reset the
CT1775 to its initialized state upon power turn-on. The MRST
(master reset) signal is provided for this purpose. Both the
flecoder and encoder, as well as all flags, are reset by a LOW
on MRST. This function interrupts and overrides all other
control signals. The MRST function can also be sued during
fault recovery routines.
DECODER OPERATION
Figure 3 illustrates the receive mode timing. Decoder detail
timing is shown in Figure 5. A receive cycle, which lasts for 20
clock periods of the 1 MHz DSC OUT, is initiated when the
decoder recognizes a valid sync and two valid Manchester data
bits. TAKE DATA goes LOW at the first HIGH to LOW
(falling edge) transition of DSC OUT, following the second
valid dat bit. COMM/DATA SYNC is updated at the next
rising edge of DSC OUT after TAKE DATA goes LOW.
COM/DATA SYNC remains in its new state until the next valid
word or until DEC RST or MRST goes LOW.
TAKE DATA remains LOW for 16 periods of DSC OUT,
during which time the 16 serial data bits are shifted into the
first rank receive register. The serial data is simultaneously
available at SERIAL DATA OUT as it is being shifted. At the
completion of decoded data shifting, TAKE DATA goes HIGH,
which transfers the data to the second rank receive register.
This data may be enabled onto the parallel data highway by
LOW on DATA SELECT at any time until the next rising edge
of TAKE DATA.
At the first rising edge of DSC OUT after TAKE DATA goes
HIGH, VALID WORD is updated. It will go LOW if the
decoded word was valid. VALID WORD will go HIGH at the
start of the next receive cycle, or after 20 microseconds if no
additional words ar received. All output flags are enabled by
VALID WORD, and therefore they are valid only as long as
VALID WORD is LOW.
TRANSCEIVER OPERATION
The CT1775 contains a transceiver similar to Aeroflex model
CT3232. When connected to a serial MUX data bus via
transformer and isolation resistors, as shown in Figure 6, the
CT1775 transceiver will fully comply with MIL-STD-1553.
The correct Technitrol part numbers for transformers used in
direct-coupled and transformercoupled operation are shown in
Figure 6.
Transceiver TX INHIBIT and RX STROBE signals are
provided to afford flexible operation. These signals may be
used to disable the transmitter and receiver, respectively.
ENCODER OPERATION
Figure 2 illustrates the transmit mode timing. Encoder detail
timing is shown in Figure 4. The transmit cycle is initiated by a
LOW on ENC ENABLE. The first HIGH to LOW (falling
edge) transition of ESC OUT, when ENC ENABLE is LOW,
starts the cycle which lasts for 20 clock periods of the 1 MHz
ESC OUT. The next LOW to HIGH transition of ESC OUT
strobes the SYNC SELECT line. A HIGH on SYNC SELECT
produces a data sync and a LOW produces a command/status
sync.
A LOW to HIGH transition of SEND DATA occurs at the
fourth falling edge of ESC OUT. This indicates the completion
of the sync interval and the start of the serial data interval.
Parallel data must be stable at the second rank transmit register
prior to the rising edge of SEND DATA, which occurs 3
microseconds (minimum) after the HIGH to LOW transition of
ENC ENABLE. LATCH DATA is used to transfer parallel data
to the first rank transmit register. LATCH DATA must be
brought LOW and DATA SELECT brought HIGH prior to the
rising edge of SEND DATA. If SEND DATA is connected
directly to LOAD DATA, it will lock out the second rank
transmit register and serial data shifting into the encoder will
proceed properly.
SCDCT1775 Rev B
5
AEROFLEX [ AEROFLEX CIRCUIT TECHNOLOGY ]
