BUS CONTROL OPERATION
To enable its use in a bus controller the ASIC has additional logic within it. This logic can be enabled by
pulling the pin labelled RT/BC low. Once the ASIC is in bus control mode, all data transfers must be
initiated by the bus control processor correctly commanding the ASIC via the subsystem interface. In bus
control mode six inputs are activated which in RT mode are inoperative and four signals with dual functions
exercise the second function (the first being for the RT operation).
To use the ACT3492 as a 1553B bus control interface, the bus control processor must be able to carry out
four basic bus-related functions. Two inputs, BCOPA and BCOPB allow these four options to be selected.
The option is then initiated by sending a negative-going strobe on the BCOPSTB input. BCOPSTB must
only be strobed low when NDRQ is high. This is particularly important when two options are required
during a single transfer.See Figure 28 and Table 1.
With these options all message types and lengths can be handled. Normal BC/RT exchanges are carried
out in option zero. This is selected by setting BCOPA and BCOPB to a zero and strobing BCOPSTB. On
receipt of the strobe, the ACT3492 loads the command word from an external latch using CWEN and H/L.
The command word is transmitted down the bus. The TX/RX bit is, however, considered as being its
inverse and so if a transmit command is sent to a RT (Figure 21), the ACT3492 in BC mode believes it has
been given a receive command. As the RT returns the requested number of data words plus its status, the
BC carries out a full validation check and passes the data into the subsystem using DTRQ, DTAK, H/L,
IUSTB and CWC as in RT operation. It also supplies GBR at the end of a valid transmission. Conversely, a
receive command sent down the bus is interpreted by the BC as a transmit command, and so the requisite
data words are added to the command word. See Figure 22.
For mode commands, where a single command word is required, option one is selected by strobing
BCOPSTB when BCOPA is high and BCOPB is low. On receiving the strobe, the command word is loaded
from the external latch using CWEN and H/L, the correct sync and parity bits are added and the word
transmitted (See Figure 23). Mode commands followed by a data word requires option two. Option two,
selected by strobing BCOPSTB while BCOPA is low and BCOPB is high, loads a data word via DWEN and
H/L, adds sync and parity and transmits them to the bus (See Figure 25). If the mode code transmitted
required the RT to return a data word, then selecting option three by strobing BCOPSTB when BCOPA and
BCOPB are both high will identify that data word and if validated, output it to the subsystem interface using
RMDSTB and H/L. This allows data words resulting from mode codes to be identified differently from
ordinary data words and routed accordingly (See Figure 24). All received status words are output to the
subsystem interface using STATSTB and H/L.
In BC option three, if the signal PASMON is active, then all data appearing on the selected bus is output to
the subsystem using STATSTB for command and status words or RMDSTB for data words. See Passive
Monitor.
RT to RT transfers require the transmission of two command words. A receive command to one RT is
contiguously followed by a transmit command to the other RT. This can be achieved by selecting option one
followed by option zero for the second command. The strobe (BCOPSTB) for option zero must be delayed
until NDRQ has gone low and returned high following the strobe for option one. The RT transmissions are
checked and transferred in the subsystem interface to the bus control processor (See Figure 26).
BC must wait for RTO to pulse before issuing subsequent messages.
PASSIVE MONITOR
The Monitor Mode may be utilized to analyze or collect all activities which occur on a selected bus. This is
initiated by selecting a bus, placing the unit in BC option three and setting PASMON low. All data
appearing on the selected bus is output to the subsystem using STATSTB for Command and Status Words
or RMDSTB for Data Words. See Figure 27.
AUTOMATED SELF-TEST
The ACT3492 has been designed to fully support a wrap-around self-test which ensures a high degree of
10
Aeroflex Circuit Technology
SCD3492 REV B 6/26/01 Plainview NY (516) 694-6700
AEROFLEX [ AEROFLEX CIRCUIT TECHNOLOGY ]
