SN75970B

SCSI DIFFERENTIAL CONVERTER-CONTROL

SLLS323A – NOVEMBER 1999 – REVISED JANUARY 2000

16

POST OFFICE BOX 655303

APPLICATION INFORMATION

To correctly set the direction of the SCSI bus signals, the SN75970B must follow the activity on the bus. An

asynchronous, 5-state controller watches the state of all the bus control signals, sets the direction of each

control signal as needed, and generates the DRVBUS and SDB outputs to control one or two external

SN75971B SCSI differential converter-data devices. The controller never generates the data driven on a bus

signal; it only enables the drivers. The clock input implements a 400-ns timer that is not part of the controller

itself. Controller-state transitions occur immediately when all the transition conditions are met. Note that the

frequency of the supplied clock, either 20 MHz or 40 MHz, must be correct in order to meet the SCSI

specifications.

As shown in Figure 8, after reset, the controller begins in the bus free state. In case the controller was attached

to an active differential bus, it waits for the SCSI bus free condition, defined as when BBSY and BSEL are

deasserted for 400 ns. While waiting for the SCSI bus free condition, the state of BBSY and BSEL passes

through to the A side. The A side bus device cannot take part in bus activity during this condition before the SCSI

bus free condition. Once SCSI bus free is detected, the SCSI arbitration state is entered. Both ABSY and BBSY

are enabled; thus when either signal asserts, both drivers turn on and both signals remain asserted until this

state is left. Normally the SCSI arbitration state ends after the winner of arbitration asserts BSEL. This would

cause the controller to go to the select 1 state. However, when BSEL is not asserted, a timeout would eventually

be detected and cause a reset of the controller. In the select 1 state two latches are open, DSEL_LATCH and

RESEL_LATCH. The first latch captures the state of BSEL so that following states can determine whether the

arbitration winner was on the A side or B side.