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SN74ACT2440 Fiches technique(PDF) 9 Page - Texas Instruments

No de pièce SN74ACT2440
Description  NuBusE INTERFACE CONTROLLER
Download  33 Pages
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Fabricant  TI1 [Texas Instruments]
Site Internet  http://www.ti.com
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SN74ACT2440 Fiches technique(HTML) 9 Page - Texas Instruments

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SN74ACT2440
NuBus
™ INTERFACE CONTROLLER
SCHS010 – D3158, OCTOBER 1988 – REVISED JANUARY 1991
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
9
slave read/write cycles
The ’ACT2440 provides all the handshake signals required to facilitate a simple NuBus
™ slave interface. In slave
applications, the local board is either written to or read from. When a NuBus
™ master wishes to access the local
board as a slave, it places the slave’s address on the bus during the start cycle. This action requires a compare
function to identify when the NuBus
™ address matches the 4-bit ID code associated with the local board. This
function is provided in the ’BCT2420 or can be built using standard MSI comparator functions. The controller
receives this input through the ID equal input (IDEQ).
Figure 8 shows the timing diagram of a typical slave read cycle. Figure 9 shows the timing diagram for a typical
slave write cycle. The slave external request status output (SEREQ) signals that the local board is being
accessed by another master. When the local board is ready to receive data and/or address information, it drives
slave grant access (SGNTA) active (low). When the local board is ready to respond to a read or write request,
it drives local acknowledge (LACK) low. The controller then issues an acknowledge on the bus, which completes
the transaction. Data and/or address information is enabled onto the local board as long as SGNTA is held low.
SEREQ does not go inactive until the first sample edge after SGNTA goes inactive. Data and/or address
information is disabled on the first sample edge after SGNTA returns inactive (high).
All slave external requests must be responded to with a local acknowledge. Allowing the NuBus
™ to timeout
does not reset the slave state machine.
higher performance slave cycles
Slave grant access (SGNTA) and local acknowledge (LACK) control the duration of slave cycles on the
’ACT2440. The simplest implementation, as previously explained, uses SEREQ, SGNTA, and LACK to form a
simple handshake. Faster slave cycles are possible by taking SGNTA low before the first sample edge after
START as shown in Figure 10. This mode of operation enables address and data information onto the local
board on the first sample edge after START. (Note: In slave-only applications, address information can be
enabled onto the local board sooner by tying AEN low on the ’BCT2440s.) As previously described, LACK
controls the completion of the slave cycle. Address and data information remains enabled onto the local board
until SGNTA returns inactive.
If the local acknowledge (LACK) and slave grant access (SGNTA) inputs are taken low before the first sample
edge after START, the acknowledge output (ACK) is generated on the next driving clock edge. This mode of
operation offers the highest performance but places the greatest demand on local circuitry.
slave lock detection
NuBus
™ locked (NLOCK) is a special output provided on the ’ACT2440 that signals when the local board is
being accessed by another master and an attention lock cycle has occurred. NLOCK informs the local board
not to modify any of its local resources until an attention null cycle is received. Figure 11 shows the timing
diagram for a slave lock-detection cycle. As shown in Figure 11, NLOCK goes active (high) when an attention
lock cycle occurs on the bus and the local board is being requested by another master. NLOCK will remain high
until the attention null cycle is received.
master block-transfer cycles
NuBus
™ 1987 master block transfers are supported by the ’ACT2440. Figure 12 shows the timing diagram for
a typical master block read. Figure 13 shows the timing diagram for a typical master block write.
A master block transfer consists of a start cycle, multiple data cycles to or from sequential address locations,
and an acknowledge cycle. The master controls the number of data words transferred and communicates this
information to the slave during the start cycle via address lines AD5–AD2. Table 2 shows the input code for
master block-transfer cycles.
During master block transfers, the slave acknowledges intermediate data cycles via the TM0 line. The ’ACT2440
detects these intermediate data cycles and generates the proper buffer control signals. The final data cycle from
the responding slave is a standard acknowledge cycle.


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