Small Computer System Interface Data Flow Control

Small Computer System Interface Data Flow Control

      Disclosed is a method for managing the data transfer rate by
monitoring the status of First In First Out (FIFO) buffer for Hard
Disk Drive (HDD) with Small Computer System Interface (SCSI)
interface.

      In SCSI interface, a target device (e.g., HDD) initiates the
data transfer with a host system.  The data transfers use REQ/ACK
handshakes with offset.  This offset specifies the maximum number of
REQ pulses that can be sent by the target in advance of the number of
ACK pulses received from the initiator.  In case of a write command,
the target must receive the number of data corresponding with the
number of asserted REQ pulses.  So the target device should control
to assert the number of REQ pulses by monitoring the empty number of
FIFO buffers.

      The Figure shows the circuit to monitor the maximum number of
REQ pulses that can be sent by the device.  This Up/Down Counter load
the number of FIFO buffers before starting the data transfer.  Then
this counter is counted down when the target device asserts a REQ
pulse and up when a data in FIFO is pulled out.  The output indicates
the number of empty FIFO buffers.  That is the number of REQ pulses
that can be sent.  So it is possible to control easily the data flow
with this simple circuit.