SN75970B

SCSI DIFFERENTIAL CONVERTER-CONTROL

SLLS323A – NOVEMBER 1999 – REVISED JANUARY 2000

17

POST OFFICE BOX 655303

APPLICATION INFORMATION

Enable ABSY and ASEL

400 ns of

BBSY = BSEL = 0?

RESET

Enable ABSY and BBSY

Set DSEL_LATCH to BSEL

Set RESEL_LATCH to AI/O

NO

YES

TIMEOUT?

YES

NO

ASEL or

(DSEL_LATCH and

BSEL)?

BBSY?

NO

NO

YES

YES

Set RESEL_LATCH to AI/O

DSEL_LATCH = 1?

NO

Enable A Side

400 ns of

BBSY = 0?

ABSY = 1 And

BBSY = 0?

NO

NO

ASEL = 1?

Enable B Side

400 ns of

ABSY = 0?

YES

YES

NO

YES

NO

YES

A

RESET

Bus Free

State

SCSI

Arbitration

State

Select 1

State

YES

Continued on Figure 9

SCSI Bus Free Condition

Select 2 State

Figure 8. Bus Free, SCSI Arbitration, and Select 1 State Flow Chart

The second latch captures the state of AI/O, this is true during a reselection phase but not during the selection

phase. When the bus is in the selection or reselection phase, the controller enters the select 1 state. There are

three possible flows depending on bus events. The first flow is that the SCSI controller on the A side won the

arbitration and asserted ASEL. In this event DSEL_LATCH would not be set. The controller passes all signals

to the B side and waits for ABSY to deassert for 400 ns, indicating that the A side controller is selecting or

reselecting a device on the B side. The object of the A side controller must be on the B side since only one device

is allowed on the A side.

The second possible flow is that DSEL_LATCH is set, indicating that the arbitration winner is on the B side, and

the winner is selecting or reselecting the device on the A side. The controller passes all signals to the A side

and waits for BBSY to deassert for 400 ns. When the A side controller responds by asserting ABSY, the controller

detects ABSY asserted and BBSY deasserts and goes to the select 2 state.