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
653Xdevice, 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
DelayIs 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