SN74ACT2440

NuBus

™ INTERFACE CONTROLLER

SCHS010 – D3158, OCTOBER 1988 – REVISED JANUARY 1991

POST OFFICE BOX 655303

4

Terminal functions (continued)

MASTER/SLAVE INPUT SIGNALS

PIN

DESCRIPTION

NAME

NO.

DESCRIPTION

IDEQ

5

ID equal. This active-low input signal is used by the slave-state machine to detect if the current NuBus

™ cycle is addressing

the local board. This input is interrogated on the sample edge (high-to-low) of the NuBus

™ clock in the cycle following the

start cycle. This input is asserted if slot and/or super-slot addresses are broadcast on the previous start cycle

LACK

50

Local acknowledge. This active-low input signal controls the NuBus

™ acknowledge signal (ACK) during slave cycles. When

the local board is ready to respond to a NuBus

™ transfer request, this input signal is driven low. The ACK output will go active

(low) on the next driving edge after LACK is sampled.

LOCTM0

LOCTM1

54

55

Local transfer-mode control. These input signals determine the sense of the NuBus

™ transfer-mode signals, TM0 and TM1,

during master start and slave acknowledge cycles. The controller latches these signals upon detecting the NuBus

™. Request

signal (NREQ). During a NuBus

™ slave acknowledge cycle, the NuBus™ TM lines reflect the current state of these inputs.

MHOLD

51

Master hold. This active-low input signal is used by the buffer control logic to hold data on the local board after the NuBus

™

cycle terminates. If this signal is true when the acknowledge cycle is received (for a NuBus

™ cycle initiated by this controller)

and the current cycle is a NuBus

™ read, then the data enable signal (DEN) remains true until MHOLD is unasserted.

Additionally, the latched TM status lines (NLTMO, NLTM1) continue to reflect the TM information presented on the NuBus

™

during the acknowledge cycle (this applies to both reads and writes). While the holding function is active, the controller inhibits

the local master from issuing another NuBus

™ start cycle when NREQ is not taken inactive after the acknowledge. In other

words, MHOLD allows only one start cycle to occur.

MLREQ

49

Master lock request. This active-low input signal, in conjunction with NREQ, causes the controller to lock the NuBus™ by

issuing an attention lock resource cycle after winning arbitration. When MLREQ is taken inactive, the controller automatically

issues a NuBus

™ attention null cycle (regardless of the state of NREQ). The attention null cycle signals the end of the locked

resource tenture.

MRDY

48

Master ready. This active-low input signal indicates to the controller that the local board is ready to perform a NuBus™ master

start cycle. The current state of the master-state machine determines this signal’s effect. If the master-state machine enters

the arbitration process (with no lock request) and wins mastership of the bus, this signal can delay issuing a start cycle for

up to 16 NuBus

™ clocks periods. After this period, the master–state machine automatically issues a NuBus™ attention null

cycle, returns to the idle state, and re-enters the arbitration process with lock request asserted, it issues an attention lock

cycle immediately upon acquiring mastership of the bus. The master-state machine then waits for MRDY to be asserted

before issuing a NuBus

™ start cycle. There is no timer in the lock mode. If the master-state machine is parked on the bus,

this signal is simply ANDed with the NuBus

™ request signal (NREQ) to generate the start cycle.

NMREQ

53

NonMaster request. This nonsynchronous active-low input asserts the NuBus

™ NonMaster request signal (NMRQ)

NREQ

47

NuBus

™ request. This active-low input signal indicates to the controller that the local board wants access to the NuBus™.

It initiates arbitration if the local board is not already the bus master.

SGNTA

56

Slave grant access. This active-low input signal indicates to the slave-state machine that the local board resources are

available. When this signal is asserted and an external request is pending, the slave-state machine issues the proper enable

signals (AEN and DEN). These enable signals remain active until SGNTA is unasserted.

SIACK

57

Slave interim acknowledge. This active-low signal indicates to the slave-state machine that an interim acknowledge (required

for block transfers) should be issued on the NuBus

™. The controller responds by asserting TM0 during block transfers.