fashion but there will be a delay of approx 70 µs between NBGT going low and DTRQ first going low.
Figures 6 and 15 show the operation of the data handshaking signals during transmit command with one
data word. 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 15.
Additional Data Information Signals
At the same time as data transfers take place, a number of information signals are made available to the
subsystem. These are INCMD, the subaddress lines SA0-SA4, the word count lines WC0-WC4 and current
word count lines CWC0-CWC4. Use of these signals is optional.
INCMD will go active low while the RT is servicing a valid command for the RT. The subaddress,
transmit/receive bit, and word count from the command word are all made available to the subsystem as
SA0-SA4, TX/RX and WC0-WC4 respectively. They may be sampled when INCMD goes low and will
remain valid while INCMD is low. See Figure 11.
The subaddress is intended to be used by the subsystem as an address pointer for the data block.
Subaddress 0 and 31 are mode commands, and there can be no receive or transmit data blocks
associated with these. (Any data word associated with a mode command uses different handshaking
operations. If the subsystem does not use all the subaddresses available, then some of the subaddress
lines may be ignored.
The TX/RX signal indicates the direction of data transfer across the RT - subsystem interface. Its use is
described in the previous section.
The word count tells the subsystem the number of words to expect to receive or transmit in a message, up
to 32 words. A word count of all 0s indicates a count of 32 words.
The current word count is set to 0 at the beginning of a new message and is incremented following each
data word transfer across the RT - subsystem interface. (It is clocked on the falling edge of the second
IUSTB pulse in each word transfer). It should be noted that there is no need for the subsystem to compare
the word count and current word count to validate the number of words in a message. This is done by the
RT.
Subsystem Use of Status Bits and Mode Commands
General Description
Use of the status bits and the mode commands is one of the most confusing aspects of MIL-STD-1553B.
This is because much of their use is optional, and also because some involve only the RT while others
involve both the RT and the subsystem.
The ACT3492 allows full use to be made of all the Status Bits, and also implements all the Mode
Commands. External programming of the Terminal Flag and Subsystem Flag Bits plus setting of the
Message Error Bit on reception of an illegal command when externally decoded is available. The
subsystem is given the opportunity to make use of Status Bits, and is only involved in Mode Commands
which have a direct impact on the subsystem.
The mode commands in which the subsystem may be involved are Synchronize, Sychronize with data
word, Transmit Vector Word, Reset and Dynamic Bus Control Acceptance. The Status Bits to which the
subsystem has access, or control are Service Request, Busy, Dynamic Bus Control Acceptance, Terminal
Flag, Subsystem Flag, and Message Error Bit. Operation of each of these Mode Commands and of the
Status Bits is described in the following sections.
Aeroflex Circuit Technology
6
SCD3492 REV B 6/26/01 Plainview NY (516) 694-6700
AEROFLEX [ AEROFLEX CIRCUIT TECHNOLOGY ]
