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CT1990

MIL-STD-1553B Remote Terminal, Bus Controller, or Passive Monitor Hybrid with Status Word Control

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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 CT1990/1 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.

With these options all message types and lengths can be handled. Normal BC/RT exchanges are carried out in

ASIC option zero. This is selected by setting BCOPA and BCOPB to a zero and strobing BCOPSTB. On receipt

of the strobe, the CT1990/1 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 by the ASIC as being its

inverse and so if a transmit command is sent to a RT (Figure 17), the ASIC 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 18.

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 20).

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 21). 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 22).

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.

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 19).

Note: For all BC operations, BCOPA and BCOPB must remain valid and stable for a minimum of 1 µs following

the leading (negative going) edge of BCOPSTB.

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.

Aeroflex Circuit Technology

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SCDCT1990 REV B 8/21/00 Plainview NY (516) 694-6700

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