PDP11 PeripheralsHbk 1972 - Trailing-Edge

a. Master sets C =00 for DATI or C=01 for DATlP, and sets
A to specify the slave address.
b. Master waits 150 ns minimum: 75 ns to allow for worst case signal skew on
the UNIBUS plus 75 ns for slave to decode address.
NOTE
This guarantees that at the output :of the slave's bus
receivers that A and C will precede
MSYN" by 75 ns min.
c. Master waits for bus to become inactive since SSYN may still be asserted from
a previous cycle.
d. Master asserts MSYN
NOTE
This condition is automatically satisfied when first becoming
bus master since SSYN must be negated before
BBSY can be asserted (see M7820 circuit).
e. When the selected slave sees its address and MSY.N, the slave device prepares
the data for transmission to the master. For devices such as memory, this
means performing a read cycle; for flip-flop registers, the data is available immediately_
f. When data is available, the slave places data on 0 and asserts
SSYN_ If the slave is a destructive read-out device, it enters a restore cycle if
the command was a DATI; for a DATIP, the slave can set a pause flag and wait
for the subsequent DATO or DATOB with the modified data before perfoming
a write cycle. The SSYN response must be made within ?5/LSec.
g. The master receives SSYN and waits 75 ns minimum to allow for skewing of
data plus any additional time the master may need for internal gating.
h. Master strobes in data from 0.
i. When the data has been accepted by the master device, it drops MSYN.
j. After 75 ns minimum wait, the master removes or alters the A and C lines. If
another DATI follows, then the master repeats the above sequence starting at
. step a. If an output operation follows, the next cycle starts at step a. of the
DATO or DA TOB operation. If this is the last bus cycle of the master's operation
and when the A and C lines are all clear, then the master clears BBSY
and relinquishes bus control.
k. The slave sees the negation of MSYN and removes data from the D lines and
negates SSYN.
1.2.3 DATO and DATOS Bus Transactions (See Figure 1·5)
Because all data transfers are with reference to the master device, a data out
(DATO) indicates data transfer from the master to the slave.
185