AND the result with 0x3FF for a byte count between zero and 944.

Step 2. Read the SCSI Status One (SSTAT1) register and examine bits [7:4], the binary representation of the number of valid bytes in the SCSI FIFO, to determine if any bytes are left in the SCSI FIFO.

Step 3. If any wide transfers have been performed using the Chained Move instruction, read the Wide SCSI Receive bit (SCSI Control Two (SCNTL2), bit 0) to determine whether a byte is left in the SCSI Wide Residue (SWIDE) register.

2.2.13 SCSI Bus Interface

The LSI53C875A performs SE transfers. TolerANT technology provides signal filtering at the inputs of SREQ/ and SACK/ to increase immunity to signal reflections.

2.2.13.1 SCSI Termination

The terminator networks provide the biasing needed to pull signals to an inactive voltage level. They also match the impedance seen at the end of the cable with the characteristic impedance of the cable. Terminators must be installed at the extreme ends of the SCSI chain and only at the ends. No system should ever have more or less than two terminators installed and active. SCSI host adapters should provide a means of accommodating terminators. The terminators should be socketed, so that if not needed they may be removed, or there should be a means of disabling them with software.

SE cables can use a 220 pull-up to the terminator power supply (Term Power) line and a 330 pull-down to ground. Due to the high performance nature of the LSI53C875A, regulated or active termination is recommended. Figure 2.5 shows a Unitrode active terminator. For additional information, refer to the SCSI-2 Specification. TolerANT technology active negation can be used with either termination network.

Note: If the LSI53C875A is used with an 8-bit SCSI bus, all 16 data lines must still be terminated or pulled HIGH.

Note: Active termination is required for Ultra SCSI synchronous transfers.

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Functional Description

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LSI 53C875A technical manual Scsi Bus Interface, Scsi Termination