If you group ports for digital output on an 8255-based device, connect only the handshaking signals of the last port in the port list, as shown in Figure 8-3.

ACK*

Port x 1

OBF*

ACK*

Port x 2

OBF*

Figure 8-3. Connecting Signal Lines for Digital Output

Types of Handshaking

Digital handshaking can be either nonbuffered or buffered. Nonbuffered handshaking is similar to unstrobed digital I/O because LabVIEW updates the digital lines immediately after every digital or handshaked pulse.

Note For the 6533 devices, LabVIEW returns immediately after storing data in its FIFO buffer.

With buffered handshaking, LabVIEW stores digital values in memory to be transferred after every handshaked pulse. Both nonbuffered and buffered handshaking transfer one digital value after each handshaked pulse. Use nonbuffered handshaking for basic digital applications. Use buffered handshaking when your application requires multiple handshaking pulses or high speeds. By using a buffer with multiple

handshaking pulses, the software spends less time reading or writing data, leaving more time for other operations.