SN75LBC970A

SCSI DIFFERENTIAL CONVERTER-CONTROL

SLLS215B – MAY 1995 – REVISED JANUARY 1999

16

POST OFFICE BOX 655303

APPLICATION INFORMATION

The third possible flow is that a device on the B side won the arbitration and is selecting or reselecting another device

on the B side. DSEL_LATCH is set, and 400 ns of BBSY is asserted first by the object of the selection or reselection.

Since ASEL is still asserted, the controller remains in the select 1 state throughout the selection or reselection. If the

BBSY deassertion is missed by the timer, again the controller remains in the select 1 state. Once the transfer state

is entered, BBSY is asserted and BSEL is dropped. This again returns the controller to a select 1 state. At the end

of the transfer both BBSY and BSEL are deasserted. After the timer limit is reached, the controller goes to the

arbitration state for the next bus arbitration.

The controller enters the select 2 state (see Figure 9) during the selection or reselection phases when the initiator

and terminator are on the opposite side of SDCC. In this state the RESEL_LATCH is closed, capturing the value of

the I/O. When RESEL_LATCH is one, reselection is indicated. When RESEL_LATCH equals zero, a selection is

indicated. RESEL_LATCH, along with the DSEL_LATCH, now defines which side the initiator is on and therefore what

direction to establish for all the bus signals. The target must be on the other side; if both target and initiator were on

the B side, the select 2 state would never be entered.

When the RESEL_LATCH is zero, indicating a selection, the connection is not made. When DSEL_LATCH is one,

the initiator is on the B side and the control lines it drives have their A side drivers enabled. These terminals are the

initiator group of ACK and ATN along with SEL. The other terminals are driven by the target and have the B side drivers

enabled. They are the target group of REQ, MSG, C/D, and I/O, along with BSY. When DSEL_LATCH is zero the

connection is reversed. Since transfer states are not started, DRVBUS is set to 1, indicating that the data transceiver

chips should not take their direction control from SDB and should be actively negated. SDB is generated from I/O

and is the bus signal that determines data transfer direction. In this case it indicates the selection phase, the controller

immediately transfers to the transfer state, where exactly the same actions are done.

When the RESEL_LATCH is 1 indicating a reselection, there is one or more actions before information states can

be entered. When the target reselects the initiator, the initiator responds by asserting BSY. Once the connection is

made, the assertion of BSY must be changed over to the target, and the controller must reverse the BSY driver

direction. It does this when SEL deasserts by transferring to the transfer state where the BSY direction is reversed.

In the select 2 state all the control line directions are set as appropriate, except that DRVBUS is not yet asserted. In

the transfer state DRBVUS is set as well.

The controller remains in the transfer state during all other SCSI states. When a bus free state is detected, it goes

back to the arbitration state to wait for the next activity. Note that after BBSY and BSEL deassert, the controller

continues to actively drive the control lines and the data lines through DRVBUS until 400 ns of continuous deassertion

is detected. The drivers are turned off only when the state change occurs.

Figure 10 shows a typical system configuration. The timeout function used in the arbitration state is implemented with

a resistor and capacitor connected to the TIMEOUT terminal. During reset and whenever the timer is not in use, the

RST can be asserted on either the A or B side, and is driven to the other side. The drive to the other side is controlled

by a bidirectional latch. When one side asserts, the other side is asserted and a latch is set to that direction. When

the first side deasserts, the driver turns off, but the direction is held until both sides are deasserted. Only then can

the direction change.