VCM-DAS-3 Reference Manual
37
Digital I/O
Signal Direction
The 24 digital I/O port signals on the VCM-DAS-3 can be configured as inputs or outputs, in
groups of eight or, in the case of port DIOC, in groups of four. Signal direction is set by
manipulating the bits in the DIOCTRL register. The logic level on any digital I/O channel can be
set by writing a 1 or 0 to the appropriate bit. The digital I/O interface of the VCM-DAS-3 are
82C55 Mode 0 compatible only.
Signal Polarity
All digital I/O circuits on the VCM-DAS-3 are non-inverted, true logic. A high logic level is
represented by a 1 in the digital I/O data registers, and a low logic level is represented by a 0.
Since Opto 22 modules invert the logic sense of signals passed through them, the register-to-
module interface is negative logic. The resulting data interface levels between the VCM-DAS-3
and I/O rack modules are shown below.
Data
Written
I/O
Pin
Output
Modules
Input
Modules
I/O
Pin
Data
Read
0
(Low)
Power On
Voltage Present
(Low)
0
1
(High)
Power Off
Voltage Absent
(High)
1
Digital Input
The logic state of the digital input channels can be read at any time by reading the DIOA, DIOB,
or DIOC registers. Simply choose the correct register and read it as an 8-bit quantity.
Digital Output
The logic state of any digital output channel can be manipulated at any time by writing to the
DIOA, DIOB, or DIOC registers. Simply choose the correct register and write the digital value as
an 8-bit quantity.
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