National Instruments 653X - Manual (Page 55)

Chapter 3 Timing Diagrams
653X User Manual 3-4 ni.com
Handshaking I/O Timing Diagrams
This section compares of the handshaking I/O protocols and includes
timing diagrams for each:
• Handshaking sequence for input operation
• State machine for input operation
• Timing specification for input operation
• Handshaking sequence for output operation
• State machine for output operation
• Timing specification for output operation
Comparing the Different Handshaking Protocols
For an overview of all handshaking protocols supported by your
653X device, see Table 3-1.
Note Whether an ACK or a REQ signal occurs first in the handshaking sequence depends
on the protocol and the direction of the transfer.
Table 3-1. Handshaking Protocol Characteristics
Protocol
REQ/ACK
Polarity
Which REQ Edge
Requests Transfer
Where the
Programmable
Delay Is Located Complementary Protocol(s)
Asynchronous Protocols
8255
Emulation Active-low Trailing Between transfers Long Pulse
Level ACK Programmable Leading Before ACK
and between transfers Level ACK
Leading-Edge Programmable Leading Before ACK
and between transfers Leading Edge
Long Pulse Programmable Leading Pulse width and
between transfers Long Pulse, 8255 Emulation,
and 8255
Trailing-Edge Programmable Trailing Pulse width and
between transfers Trailing-Edge