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Chapter 4
Theory of Operation
©
National Instruments Corporation
4-13
All three ports on the 82C55A are TTL-compatible. When enabled, the
digital output ports are capable of sinking 2.5 mA of current and sourcing
2.5 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.