Drive Select; Signal Values; Or-Tied Signals; Signal Sources - Seagate ST11200 Product Manual

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DIFFSENS (Differential Sense) - When the drive has differential SCSI I/O circuits, the DIFFSENS signal
disables the drive's differential driver/receiver circuits if the SCSI I/O cable is plugged in upside down, or if a
single-ended SCSI I/O cable is plugged into a differential I/O drive. Disabling the differential I/O drivers/
receivers is necessary to prevent burning them out if a grounded I/O line is connected to any of the differential
circuit outputs, which are at a positive voltage (+2 V or +3 V) when not disabled.
DB(7-0,P) and DB(15-8,P1) (Data Bus) - Sixteen data bit signals, plus parity bit signals form a Data Bus.
DB(7) is the most significant bit and has the highest priority during the Arbitration phase (on both eight and
sixteen device systems). Bit number significance, and priority decrease downward to DB(0), and then from
DB15 down to DB8 (DB0 is higher than DB15). A data bit is defined as one when the signal is asserted and
is defined as zero when the signal is negated.
Data parity DB(P) and DB(P1) is odd - The use of parity is a system option. The disc drive always checks
parity on the data bits, but has the capability to enable/disable parity error reporting to the host. See configu-
ration selection in the applicable Product Manual. Parity checking is not valid during the Arbitration phase.
Greater detail on each of the SCSI Bus signals is found in the following sections.
2.1.1

Drive Select

For SCSI ID selection install drive select jumpers as shown in configuration selection figure in applicable
Product Manual. Refer to section 10 of the individual drive Product Manual for the location of the drive select
header. The disc drive using the eight bit data interface can have one of eight ID bits selected by installing 0
to 3 jumpers in a binary coded configuration on the drive select header. Drives using the 16 bit data interface
can have one of sixteen ID bits selected by installing 0 to 4 jumpers in a binary coded configuration on the
drive select header.
2.1.2

Signal Values

Signals may assume true or false values. There are two methods of driving these signals. In both cases, the
signal shall be actively driven true, or asserted. In the case of OR-tied drivers, the driver does not drive the
signal to the false state, rather the bias circuitry of the bus terminators pulls the signal false whenever it is
released by the drivers at every SCSI device. If any driver is asserted, then the signal is true. In the case of
non-OR-tied drivers, the signal may be negated. Negated means that the signal may be actively driven false,
or may be simply released (in which case the bias circuitry pulls it false), at the option of the implementor.
2.1.3

OR-Tied signals

The BSY and RST signals shall be OR-tied only. In the ordinary operation of the bus, these signals are
simultaneously driven true by several drivers. No signals other than BSY, RST, and DB(P) are simultaneously
driven by two or more drivers, and any signal other than BSY and RST may employ OR-tied or non-OR-tied
drivers. DB(P) shall not be driven false during the Arbitration phase. There is no operational problem in
mixing OR-tied and non-OR-tied drivers on signals other than BSY and RST.
2.1.4

Signal sources

Table 2.1.4-1 indicates which type of SCSI device is allowed to source each signal. All SCSI device drivers
that are not active sources shall be in the passive state. Note that the RST signal may be sourced by any
SCSI device at any time. The disc drive functions as a target.
Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E