8
SCDCT1990 Rev C
BUS DRIVER/RECEIVER INTERFACE
Receive Data
The decoder chip requires two TTL signals, RXDATA and
RXDATA
, to represent the data coming in from the
bus. PDIN should be driven to a logic level ‘1’ when the bus waveform exceeds a specified positive threshold and
NDIN should be driven to a logic level ‘1’ when a specified negative threshold is exceeded. During the quiet
period on the bus both signals should be at the same logic level. All the bus receivers must be permanently
enabled, the selection of the bus in use is controlled within the ASIC.
Transmit Data
The signals generated by the encoder chip, TXDATA and
TXDATA
, are of the same format as the receive data.
The only difference is that the TTL signals are negative logic, e.g. the signal is active when on logic level "0". This
means that when the encoder is quiet both TXDATA and
TXDATA
are at logic level "1". Both the signals should
be used in conjunction with
TXINHIBIT 0
and
TXINHIBIT 1
. TX INHIBIT 0 and TX INHIBIT 1 enable the
appropriate driver when it should be transmitting. Figure 5 shows an example of a typical interface circuit between
the CT1990/1 and a driver/receiver unit.
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
.