CT1995 Datasheet, PDF (3/25 Pages) Aeroflex Circuit Technology – MIL-STD-1553B Remote Terminal, Bus Controller, or Passive Monitor Hybrid with Status Word Control

REMOTE TERMINAL OPERATION
Receive Data Operation
All valid data words associated with a valid receive data command word for the RT are passed to the subsystem. The
RT examines all command words from the bus and will respond to valid (i.e. correct Manchester, parity coding etc.)
commands which have the correct RT address (or broadcast address if the RT broadcast option is enabled). When the
data words are received, they are decoded and checked by the RT and, if valid, passed to the subsystem on a word by
word basis at 20 µs intervals. This applies to receive data words in both Bus Controller to RT and RT to RT messages.
When the RT detects that the message has finished, it checks that the correct number of words have been received
and if the message is fully valid, then a Good Block Received signal is sent to the subsystem, which must be used by
the subsystem as permission to use the data just received.
The subsystem must therefore have a temporary buffer store up to 32 words long into which these data words can be
placed. The Good Block Received signal will allow use of the buffer store data once the message has been validated.
If a block of data is not validated, then Good Block Received will not be generated. This may be caused by any sort of
message error or by a new valid command for the RT being received on another bus to which the RT must switch.
Transmit Data Operation
If the RT receives a valid transmit data command addressed to the RT, then the RT will request the data words from the
subsystem for transmission on a word by word basis. To allow maximum time for the subsystem to collect each data
word, the next word is requested by the RT as soon as the transmission of the current word has commenced.
It is essential that the subsystem should provide all the data words requested by the RT once a transmit sequence has
been accepted. Failure to do so will be classed by the RT as a subsystem failure and reported as such to the Bus
Controller.
Control of Data Transfers
This section describes the detailed operation of the data transfer mechanism between the RT and subsystems. It
covers the operations of the signals DTRQ, DTAK, IUSTB, H/L, GBR, NBGT, TX/RX during receive data and transmit
data transfers.
Figure 7 shows the operation of the data handshaking signals during a receive command with two data words. When
the RT has fully checked the command word, NBGT is pulsed low, which can be used by the subsystem as an
initialization signal. TX/RX will be set low indicating a receive command. When the first data word has been fully
validated, DTRQ is set low. The subsystem must then reply within approximately 1.5 µs by setting DTAK low. This
indicates to the RT that the subsystem is ready to accept data. The data word is then passed to the subsystem on the
internal highway IH08-IH715 in two bytes using IUSTB as a strobe signal and H/L as the byte indicator (high byte first
followed by low byte). Data is valid about both edges of IUSTB. Signal timing for this handshaking is shown in
Figure 12.
If the subsystem does not declare itself busy, then it must respond to DTRQ going low by setting DTAK low within
approximately 1.5 µs. Failure to do so will be classed by the RT as a subsystem failure and reported as such to the Bus
Controller.
It should be noted that IUSTB is also used for internal working in the RT. DTRQ being low should be used as an enable
for clocking data to the subsystem with IUSTB.
Once the receive data block has finished and been checked by the RT, GBR is pulsed low if the block is entirely correct
and valid. This is used by the subsystem as permission to make use of the data block. If no GBR signal is generated,
then an error has been detected by the RT and the entire data block is invalid and no data words in it may be used.
If the RT is receiving data in an RT to RT transfer, the data handshaking signals will operate in an identical fashion but
there will be a delay of approx 70 µs between NBGT going low and DTRQ first going low. See Figure 10.
Figure 6 shows the operation of the data handshaking signals during transmit command with three data words. As with
the receive command discussed previously, NBGT is pulsed low if the command is valid and for the RT. TX/RX will be
set high indicating a transmit data command. While the RT is transmitting its status word, it requests the first data word
from the subsystem by setting DTRQ low. The subsystem must then reply within approximately 13.5 µs by setting
DTAK low. By setting DTAK low, the subsystem is indicating that it has the data word ready to pass to the RT. Once
DTAK is set low by the subsystem, DTRQ should be used together with H/L and TX/RX to enable first the high byte and
then the low byte of the data word onto the internal highway IH08-IH715. The RT will latch the data bytes during IUSTB,
and will then return DTRQ high. Data for each byte must remain stable until IUSTB has returned low. Signal timing for
this handshaking is shown in Figure 11.
Aeroflex Circuit Technology
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SCDCT1995 REV A 11/21/01 Plainview NY [phone]