User manual

19 Chapter 3
3.2.3 Input/Output Control
A control word can be written to a port's configuration register (Base+3,
7, 11 and 15 respectively for ports 0, 1, 2 and 3 on the PCI-1753, and
Base+35, 39, 43 and 47 respectively for ports 0, 1, 2 and 3 on the
PCI-1753E) to set the port as an input or an output port, unless the ports
are set as output ports via jumpers (refer to Section 2.3, Jumper Settings).
Table 3-1 shows the format of a control word.
3.2.4 Initial Configuration
The initial configuration of each port depends on the input/output jumper
setting of each port, on the setting of the jumper JP1, and on whether the
power was actually disconnected or whether the system was hot reset.
If jumper JP1 is not enabled, all ports configured by software are
automatically set as input ports during system start up or reset, with a
default signal level of logic 1 (high). All ports set via jumpers as output
ports are set as output ports during system start up or reset, signal level
logic 0 (0 V).
Table 3.1: Bit map of port configuration register
D7 D6 D5 D4 D3 D2 D1 D0
Not
read
Not
read
Not
read
PortA
0: output
1: input
Port C
higher
bits
0: output
1: input
Not
read
Port B
0: output
1: input
Port C
lower bits
0: output
1: input
Note A control word has no effect if the corresponding
port is set as an output port by a jumper.
Warning Before setting any port as an output port via
software, make sure that a safe output value has
also been set. An output voltage will appear at
the pins immediately following the control word
taking effect. If no output value was specified,
the value will be indeterminate (either 0 or 1),
which may cause a dangerous condition.