
Signal Descriptions
2-7
The SYM53C141 receives data and parity signals and passes them from
the source bus to the load bus and provides any necessary edge shifting
to guarantee the skew budget for the load bus. Either side of the
SYM53C141 can be the source bus or the load bus. The side asserting,
deasserting or releasing the SCSI signals is the source side. The
following steps are a part of the SYM53C141 data path:
1.
Asserted data is accepted from the receiver logic as soon as it is
received. Once the clock signal has been received, data is gated
from the receiver latch.
2.
The path is tested to ensure the signal, if being driven by the
SYM53C141, is not misinterpreted as an incoming signal.
3.
The data is then leading edge filtered. The assertion edge is held for
a specified time to prevent any signal bounce. The duration is then
controlled by the input signal.
4.
The next stage is a latch that samples the signal. This provides a
stable data window for the load bus.
5.
The final stage develops pull-up and pull-down controls for the SCSI
I/O logic, including 3-state controls for the pull-up.
6.
A parallel function ensures that bus (transmission line) recovery is
ensured for a specified time after the last signal deassertion on each
signal line.
2.7.2 Busy (BSY) Control
A_SBSY/ and B_SBSY
±
signals are propagated from the source bus to
the load bus. These signals go through the following processing steps:
1.
The path is tested to ensure the signal, if being driven by the
SYM53C141, is not misinterpreted as an incoming signal.
2.
The data is then leading edge filtered. The assertion edge is held for
a specified time to prevent any signal bounce. The duration is then
controlled by the input signal.
3.
The next stage has two modes. One mode simply passes data
through. The other mode behaves like a large filter. The current state
in the SYM53C141 state machine that tracks SCSI phases selects
the mode. The large filter mode is used where the Busy (BSY) and
Select (SEL) sources may switch from side to side. This output is
then fed to the output driver which is a pull-down open collector only.