Chapter 3
Timing Diagrams
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National Instruments Corporation
3-5
For the NI 653
X
to communicate with peripheral devices in handshaking
mode, you must verify the following items:
•
You are using complementary protocols. For example, use
8255-emulation protocol with long-pulse protocol.
•
The ACK/REQ polarity are the same. For example, 8255 emulation
is active low only, so the other device must use the long-pulse protocol
and have active low ACK/REQ polarity.
Using the Burst Protocol
Burst protocol is a synchronous, or clocked, protocol. In addition to using
the ACK and REQ signals like the other handshaking protocols, in burst
protocol, the NI 653
X
and the peripheral device share a clock signal over
the PCLK line.
The NI 653
X
asserts the ACK signal if it is ready to perform a transfer. If
the peripheral device also asserts the REQ signal indicating it is ready,
a transfer occurs on the rising edge of the PCLK signal. Refer to
Figures 3-3 and 3-4 for examples of burst protocol transfers. Dashed lines
indicate when data is transferred.
Synchronous Protocol
Burst
Programmable
Neither (level REQ)
Clock speed
Burst
* Asynchronous protocols can compensate automatically to cable length, yet for synchronous protocols, select an
appropriate speed for your cable when configuring your device.
Select a delay of at least the following:
•
0 for a typical cable up to 1 m
•
1 (70 ns) for a typical cable up to 5 m
•
2 (140 ns) for a typical cable up to 15 m long
Table 3-1.
Handshaking Protocol Characteristics (Continued)
Protocol
REQ/ACK
Polarity
Which REQ Edge
Requests Transfer
Where the
Programmable
Delay Is Located
Complementary
Protocol(s)
