User`s manual

Chapter 3 Function Description 19
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.
Table 3-1: Bit map of port configuration register
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 soft-
ware, 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.
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).
D7 D6 D5 D4 D3 D2 D1 D0
Not
read
Not
read
Not
read
Port A
0: output
1: input
Port C
upper bits
0: output
1: input
Not
read
Port B
0: output
1: input
Port C
lower bits
0: output
1: input