SN74ACT2440

NuBus

™ INTERFACE CONTROLLER

SCHS010 – D3158, OCTOBER 1988 – REVISED JANUARY 1991

POST OFFICE BOX 655303

11

maximum block-transfer performance

As a master, the ’ACT2440 is capable of supporting the maximum block transfer rate of 37.6M-bytes/second

(one start cycle followed by 16 consecutive 100-ns data cycles). Figure 12 shows a more typical situation where

the slave controls the block transfer rate via the intermediate acknowledge signal (TM0). Note that the ’ACT2440

generates a data clock (DCLK) every clock cycle that TM0 is low. The final data cycle is a normal acknowledge

cycle.

In slave block transfer mode, the ’ACT2440 has been designed to provide a simple handshake between the

slave interim acknowledge (SIACK) input and the slave external request (SEREQ) output as shown in

Figure 15. Note that each data clock (DCLK) cycle goes high for 100 ns as a result of the simple handshake

between SIACK and SEREQ. In this simpler mode of operation, the maximum intermediate data transfer rate

when using the ’ACT2441 is 200 ns, which equates to approximately 20M-bytes/second.

NuBus

™ cycles from the parked position

As long as RQST remains unasserted, the bus owner is considered to be parked on the bus and may continue

to use the bus without the necessity of going through an arbitration contest in which it is the only contender. The

ANSI/IEEE 1196-1987 specification requires that as soon as another module drives the RQST line asserted,

an arbitration contest is started and the present bus owner (currently parked on the bus) must not begin another

transaction. The concept of bus parking reduces the average time needed to acquire the bus in systems with

a small number of active contenders.

When using the ’ACT2440 NuBus

™ controller from a parked position, the local board does not know if it remains

the NuBus

™ master and begins another transaction until the START signal has been generated. In other words,

just because the local board has taken MRDY and NREQ active (low), does not mean the ’ACT2440 continues

to own the bus and has generated a START cycle.

When the ’ACT2440 is in the parked position (NMSTR high) and no other masters are requesting the bus, a start

cycle is generated on the driving edge after NREQ and MRDY are taken active (low).

Figure 16 shows a situation where an old NuBus

™ master is initially parked on the bus and is attempting to issue

another START cycle (by taking MRDY low); but loses to a new master who is attempting to access data from

resources that are available on the old master’s board. In other words, the new master wins the bus and is trying

to use the old master as a slave. This situation is similar to the local resource conflict timing diagram shown in

Figure 6.

In Figure 16, the old master learns that it has lost the bus by detecting that NMSTR has gone inactive (low) during

the start cycle. The new master, which has just won the bus and has generated a start cycle, is attempting to

access data from the old master. The slave external request (SEREQ) output on the old master detects this

access request by going active (low) on the first sample edge after the start cycle. At this time, the old master

may want to take MRDY back to the inactive level (as shown in Figure 16) so that it has control of the START

signal after winning back the bus. If MRDY is not taken back to the inactive level (high) after losing the bus, then

the ’ACT2440 immediately issues a start cycle after the acknowledge cycle has been generated.

If the new master was directing the access cycle at a different slave, then the SEREQ output on the old master

would remain inactive (high) and the MRDY input on the old master can be kept low in order to generate a start

cycle as soon as the old master wins back the bus.

Notice from the timing diagram that if the old master takes MRDY low at the same time or in the following cycle,

then the old master loses to the new master.

If the old master takes MRDY low on the cycle before the new master takes RQST low, then the old master

retains the bus and completes its cycle.