National Instruments NI-DNET - Manual (Page 33)

Chapter 4 NI-DNET Programming Techniques
NI-DNET User Manual 4-2 ni.com
When you select an ExpPacketRate for an I/O connection, you must
consider all I/O connections in your system. For example, although you
might be able to configure an
ExpPacketRate of 3 ms for a single
I/O connection, you cannot configure a 3 ms ExpPacketRate for 40 I/O
connections because DeviceNet’s bandwidth capabilities cannot support
40 messages in a 3 ms time frame.
The following sections describe how to evaluate system considerations so
that you can configure valid values for
ExpPacketRate .
Strobed I/O
For strobed I/O connections, the master broadcasts a single strobe
command message to all strobed slaves. Since all strobed I/O connections
transfer data at the rate of this single strobe command message, the
ExpPacketRate of each strobed I/O connection must be set to the
same value.
The common ExpPacketRate for all strobed I/O connections should
provide enough time for the strobe command and each strobed slave’s
response. You must also allow time for other I/O messages and explicit
messages to occur in the
ExpPacketRate time frame. If you do not know
the time needed, let NI-DNET calculate a safe value for you (refer to the
section
Automatic EPR Feature later in this chapter).
Figure 4-1 shows a timing example for four strobed devices at MAC ID 9,
11, 12, and 13. Notice that since MAC ID 11 is slow to respond, the
ExpPacketRate is set to 20 ms to provide additional safety margin for
other messages.
Figure 4-1. Strobed I/O Timing Example
Strobe Command Strobe Command Strobe Response 9 StrobeResponse 13 Strobe Response 12 Strobe Response 11
0 ms 5 ms 10 ms 20 ms 15 ms