Instruction manual

Selection of Fast or Slow SCSI
Provision has been made to allow operator selection of Fast SCSI transfer rates,
through nonvolatile flags setable through the console. Software and Firmware
can inspect these flags, one per SCSI bus, to determine the maximum transfer
rate (5 or 10 MB/s). This is done through the use of the SCSI Synchronous Data
Transfer Rate negotiation. At time of manufacture, the flags will default to Slow
SCSI to ease a system upgrade where existing SCSI devices and cables are to be
used. The operator can enable Fast SCSI once the SCSI bus configuration has
been reviewed and found that it meets Digital’s criteria for operation at that data
rate. The bits in these locations are zeros for the 300/400/500 models.
This feature appears to software simply as one NVR bit per SCSI bus, intended
to be used to provide guidance about SCSI transfer rates when the hardware
supports Fast SCSI. It does not prevent the mandatory update of operating
system SCSI code to support the 40 MHz oscillator.
The 53CF94-2 requires reconfiguration after a hardware reset for TURBOchannel
or a software reset for the 53CF94-2. The 53CF94-2 has four configuration
registers (see Section 8.4) that must be configured:
07 000000 0707
MR−0104−93RAGS
00010BID 00001PAR XXXXXB00
Config 1 Config 2 Config 3
Field Value Description
BID 110 Host bus ID
PAR 111 Parity checking configurations
B 1 Save residual byte
FF 11 Fast_clk/Fast_SCSI
N 1 Enable_active_negation
T 0 Transfer_count_test_mode
E 1 Back-to-back_transfer_enable
X Reserved
Save Residual Byte Bit
If this bit (see Section 8.2.6) is clear (0), the last byte send with an odd
transfer count will be transferred with a meaningless byte. However, both
bytes are considered data and the meaningless byte may be included in a
DMA transaction to memory. Set the Save Residual Byte bit to prevent
this inclusion.
The clock conversion factor register in the 53CF94-2 must be written with the
correct value for the appropriate clock speed.
I/O Programming 9–17