Figure 2.4 LSI53C875A Host Interface SCSI Data Paths

Asynchronous

 

Asynchronous

 

Synchronous

SCSI Send

 

 

 

SCSI Receive

 

SCSI Send

 

 

 

 

 

 

 

 

 

 

 

PCI Interface**

 

PCI Interface**

 

PCI Interface**

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DMA FIFO*

 

DMA FIFO*

 

DMA FIFO*

(8 Bytes x 118)

 

(8 Bytes x 118)

 

(8 Bytes x 118)

 

 

 

 

 

SWIDE Register

 

 

 

 

 

 

 

 

SODL Register*

 

SIDL Register*

 

 

SODL Register*

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SCSI Interface**

 

SCSI Interface**

 

SODR Register*

 

 

 

 

 

 

 

 

 

 

 

 

SCSI Interface**

Synchronous

SCSI Receive

PCI Interface**

DMA FIFO*

(8 Bytes x 118)

SWIDE Register

SCSI FIFO**

(1 or 2 Bytes x 31)

SCSI Interface**

*= No parity protection ** = Parity protected

The following steps determine if any bytes remain in the data path when the chip halts an operation:

Asynchronous SCSI Send –

Step 1. If the DMA FIFO size is set to 112 bytes (bit 5 of the Chip Test Five (CTEST5) register cleared), look at the DMA FIFO (DFIFO) and DMA Byte Counter (DBC) registers and calculate if there are bytes left in the DMA FIFO. To make this calculation, subtract the seven least significant bits of the DBC register from the 7-bit value of the DFIFO register. AND the result with 0x7F for a byte count between zero and 112.

If the DMA FIFO size is set to 944 bytes (bit 5 of the Chip Test Five (CTEST5) register is set), subtract the 10 least significant

SCSI Functional Description

2-29

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LSI technical manual LSI53C875A Host Interface Scsi Data Paths, Asynchronous Scsi Send