Theory of Operation
Chapter 3
AT-MIO-16D User Manual
3-12
© National Instruments Corporation
I/O Connector
A
Digital
Input
Register
B
EXT STROBE WR*
EXTSTROBE*
/
4
/
4
/
4
/
4
DO REG WR
DOUT0 ENABLE
/
4
DATA <3..0>
DATA <7..4>
/
4
DOUT1 ENABLE
DIREG RD
DATA <7..0>
/
8
BDIO <3..0>
ADIO <3..0>
PC AT I/O Channel
DOUT1
Digital
Output
Register
DOUT0
Digital
Output
Register
Figure 3-5. Digital I/O Circuitry Block Diagram
The digital I/O lines are controlled by the Digital Output Register and monitored by the Digital Input
Register. The Digital Output Register is an 8-bit register that contains the digital output values for
both ports 0 and 1. When port 0 is enabled, bits <3..0> in the Digital Output Register are driven
onto digital output lines ADIO<3..0>. When port 1 is enabled, bits <7..4> in the Digital Output
Register are driven onto digital output lines BDIO<3..0>.
Reading the Digital Input Register returns the state of the digital I/O lines. Digital I/O lines
ADIO<3..0> are connected to bits <3..0> of the Digital Input Register. Digital I/O lines
BDIO<3..0> are connected to bits <7..4> of the Digital Input Register. When a port is enabled,
the Digital Input Register serves as a read-back register, returning the digital output value of the
port. When a port is not enabled, reading the Digital Input Register returns the state of the digital
I/O lines as driven by an external device.
Both the digital input and output registers are TTL-compatible. The digital output ports, when
enabled, are capable of sinking 24 mA of current and sourcing 2.6 mA of current on each digital
I/O line. When the ports are not enabled, the digital I/O lines act as high-impedance inputs.
The external strobe signal EXTSTROBE*, shown in Figure 3-5, is a general-purpose strobe
signal. Writing to an address location on the AT-MIO-16D board generates an active-low 200-nsec
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