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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

.

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 

 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