National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ User Manual download pdf (Page 70)

Chapter 4 Theory of Operation

All three ports on the 8255A are TTL-compatible. When enabled, the digital output ports are

capable of sinking 2.4 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.

Timing I/O Circuitry

The Lab-PC+ uses two 8253 Counter/Timer integrated circuits for data acquisition timing and for

general-purpose timing I/O functions. One of these is used internally for data acquisition timing,

and the other is available for general use. Figure 4-6 shows a block diagram of both groups of

timing I/O circuitry (counter groups A and B).

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National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ User Manual Page 70