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Issue
99, October 1998
Networking
with DeviceNet—Part
2: A Weather Station Application
by
Jim Brady
Start
• Can
Chips
• Chip
Setup •
Real Time
• Message
Flow
• Connections
• Timers
• Analog
Input Point
• Identity
Object
• Fragmented
Messages
• Getting
Physical
• Applying
DeviceNET
• Software,Sources
TIMERS
For
each connection, you need a time-out timer. You also need
a timer for sending fragments. The BIOS clock is handy,
but who wants to deal with 18.2 Hz?
With
the Micro/sys board, the 25-MHz system clock is divided
by 21 to drive timer 0 in the ’386EX. Timer 0 further
divides by 65,536, producing 18.2 Hz. Loading 0xE884 into
the timer 0 count register resulted in BIOS clock interrupts
at a more friendly rate of 20.0 Hz.
The
connection time-out time depends on the expected packet
rate, which is set by the master. When a DeviceNet message
comes in, I reload the timer for that connection, and
my timer interrupt handler then decrements it at a 20.0-Hz
rate. If it reaches zero before another message comes
in, the connection times out.
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