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SCSI Operating Registers
SYM53C875/875E Data Manual
5-15
Register 06 (86)
SCSI Destination ID (SDID)
Read/Write
Bits 7-4 Reserved
Bits 3-0 E ncoded Destination SCSI ID
Writing these bits sets the SCSI ID of the
intended initiator or target during SCSI rese-
lection or selection phases, respectively. When
executing SCRIPT S, the SCRIPT S processor
writes the destination SCSI ID to this register.
T he SCSI ID is defined by the user in a
SCRIPT S SELECT or RESELECT instruc-
tion. T he value written should be the binary-
encoded ID value. T he priority of the 16 possi-
ble IDs, in descending order, is:
Register 07 (87)
General Purpose (GPREG)
Read/Write
Bits 7-5 Reserved
Bits 4-0 GPIO4-GPIO0 (General Purpose)
T hese bits can be programmed through the
GPCNT L Register to become inputs, outputs
or to perform special functions. As an output,
these pins can be used to enable or disable
external terminators. T hese signals can also be
programmed as live inputs and sensed through
a SCRIPT S Register to Register Move Instruc-
tion. GPIO(3-0) default as inputs and GPIO4
defaults as an output pin. When configured as
inputs, an internal pull-up is enabled.
GPIO4 can be used to enable or disable V
PP
,
the 12-volt power supply to the external flash
memory. T his bit powers up with the power to
the external memory disabled.
SDMS Use of GPIO Pins
Symbios SDMS software uses GPIO3 to
detect a differential board. If the pin is pulled
low externally, the board will be configured by
SDMS as a differential board. If it is pulled
high or left floating, SDMS will configure it as
a single-ended board. T he Symbios PCI to
SCSI host adapters use the GPIO4 pin in the
process of flashing a new SDMS ROM.
Symbios SDMS software uses the GPIO0 pin
to toggle SCSI device LEDs, turning on the
LED whenever the SYM53C875 is on the
SCSI bus. SDMS drives this pin low to turn on
the LED, or drives it high to turn off the LED.
SDMS uses the GPIO1-0 pins to support
serial EEPROM access. When serial EEPROM
access is enabled, GPIO1 is used as a clock and
GPIO0 is used as data.
RES
7
RES
6
RES
5
RES
4
ENC3
3
ENC2
2
ENC1
1
ENC0
0
Default>>>
X
X
X
X
0
0
0
0
Highest
4
Lowest
12
7
6
5
3
2
1
0
15
14
13
11
10
9
8
RES
7
RES
6
RES
5
GPIO4
4
GPIO3
3
GPIO2
2
GPIO1
1
GPIO0
0
Default>>>
X
X
X
0
X
X
X
X