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Revision 4.2

July 29, 2005

Confidential

VSC7177

Data Sheet

Sideband-Based Port Selection

External I/O pins are used to configure port connectivity when the sideband-based port selection modes are active.

The protocol-based port selection mode is disabled when sideband-based port selection modes are enabled.

There are two sideband-based port selection operating modes. One mode selects the active port connected to port 2

based on the logical value of the PORTSEL0 input. Port 0 is the active port connected to port 2 when PORTSEL0 is

LOW. Port 1 is the active port connected to port 2 when PORTSEL0 is HIGH. This level-sensitive sideband mode is

enabled when MODE1 is LOW and PORTSEL1 is LOW. Note that the requirement to have PORTSEL1 LOW in

level-sensitive sideband mode does not exist in the VSC7173 and VSC7175. For a complete list of differences

between the VSC7177 and the VSC7173 and the VSC7175, see “Pinout Differences Between VSC7177 and

VSC7173/VSC7175,” page 13.

The other sideband-based mode chooses the active port connected to port 2 when rising edge transitions are detected

on the PORTSEL0 and PORTSEL1 input pins. This edge-sensitive sideband-based mode is enabled when MODE1 is

HIGH. Port 0 is selected as the active port connected to port 2 when a rising edge transition is detected on

PORTSEL0. PORTSEL1 can be in a HIGH or LOW state during a transition of PORTSEL0. Port 1 is selected as the

active port when a rising edge transition is detected on PORTSEL1. PORTSEL0 can be in a HIGH or LOW state

during a transition of PORTSEL1. The VSC7177 grants access to the last PORTSELx signal that transitions from

LOW to HIGH.

To allow redundant hosts to access single-port drives in edge-sensitive sideband port selection mode, two inputs,

PORTSEL0 and PORTSEL1, control the multiplexer to the drive port and the enable signal to their output port.

Because of this, each host needs to gain control of the VSC7177 if the other host fails. It is assumed that if a host fails,

it will drive PORTSELx HIGH, LOW, or open, but that it will not be transitioning the signal. It is also assumed that

the two hosts have a host-to-host communication path that allows them to agree which host should access the drive at

any time. If the host at port 1 fails, the host at port 0 can regain access to the drive by first driving PORTSEL0 LOW,

then driving it HIGH. Essentially, the last host to transition its PORTSELx signal from LOW to HIGH is granted

access. This simple scheme ensures that a healthy host can always recover access to the drive at port 2 if the other host

fails.

Status Signals—Sideband Mode

The P0SLTD output reports the OOB status of port 1 (OOBPORT1) when the device is in level-sensitive sideband-

based port selection mode. The P0SLTD output indicates the active port when the device is in edge-sensitive

sideband-based port selection mode. P0SLTD is LOW when port 1 is active and HIGH when port 0 is active.

The OOBPORT0 output reports the state of the OOB detector from port 0’s input receiver. Table 2 contains a

summary of this information. For more information about the OOBPORT0 and OOBPORT1 signals, see “OOB

Transfer,” page 12.

Table 2. Output Status in Sideband Port Selection Mode

Sideband Port

Selection Mode

Port Connected

(to Port 2)

Output Pins

P0SLTD

OOBPORT0

Level-sensitive

Port 0 or 1

OOBPORT1

OOBPORT0

Edge-sensitive

Port 0

1

OOBPORT0

Edge-sensitive

Port 1

0

OOBPORT0