DP8390D datasheet(36/56 Pages) NSC

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130 Bus Arbitration and Timing (Continued)

REMOTE READ TIMING

1) The DMA reads byteword from local buffer memory and

writes byteword into latch increments the DMA address

and decrements the byte count (RBCR01)

2) A Request Line (PRQ) is asserted to inform the system

that a byte is available

3) The system reads the port the read strobe (RACK)is

used as an acknowledge by the Remote DMA and it goes

back to step 1

Steps 1 – 3 are repeated until the remote DMA is com-

plete

Note that in order for the Remote DMA to transfer a byte

from memory to the latch it must arbitrate access to the

local bus via a BREQ BACK handshake After each byte or

word is transferred to the latch BREQ is dropped If a Local

DMA is in progress the Remote DMA is held off until the

local DMA is complete

TLF8582 – 72

REMOTE WRITE TIMING

A Remote Write operation transfers data from the IO port

to the local buffer RAM The NIC initiates a transfer by re-

questing a byteword via the PRQ The system transfers a

byteword to the latch via IOW this write strobe is detected

by the NIC and PRQ is removed By removing the PRQ the

Remote DMA holds off further transfers into the latch until

the current byteword has been transferred from the latch

PRQ is reasserted and the next transfer can begin

1) NIC asserts PRQ System writes byteword into latch

NIC removes PRQ

2) Remote DMA reads contents of port and writes

byteword to local buffer memory increments address

and decrements byte count (RBCR01)

3) Go back to step 1

Steps 1 – 3 are repeated until the remote DMA is com-

plete

TLF8582 – 73

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