|
Issue
99, October 1998
Networking
with DeviceNet—Part
2: A Weather Station Application
by
Jim Brady
Think
programming a DeviceNet interface in C++ is tough? Jim
disagrees. With its excellent response times and adequate
program size, Jim gets the same excitement writing code
for a fast 32-bit CPU as he got from his ’67 Camaro.
Start
• Can
Chips
• Chip
Setup •
Real Time
• Message
Flow
• Connections
• Timers
• Analog
Input Point
• Identity
Object
• Fragmented
Messages
• Getting
Physical
• Applying
DeviceNET
• Software,Sources
If
you like programming as much as I do, you’re in for
a real treat with DeviceNet. The DeviceNet specification
is fully object-oriented, with each object described in
terms of attributes and services.
These
items correspond to C++ class data and member functions,
so if you use C++, all you have to do is understand the
specification and translate it to code. Just make sure
you have some strong coffee on hand when tackling the
tricky parts. I had the most trouble—err…fun—with
connection states and fragmented messaging.
Let’s
cover the PC/104 hardware first. After surveying the many
processor boards available for PC/104, I went with the
Micro/sys SBC1386, a 25-MHz ’386EX board, shown in
Photo 1. It comes with BIOS and a DOS run-time environment
that runs the application out of RAM. That way, you don’t
need a special library and linker to generate ROMable
code.
|
Photo 1—The
PC/104 weather station is entirely powered from
the DeviceNet bus. The weather station board
is sandwiched between the ’386EX CPU board
on bottom and the DeviceNet interface on top.
The humidity transducer and thermistor are on
the small board in front.
|
The
board also includes the Borland remote debugger in flash
memory. It’s nice to be able to send the program
to the board at 115 kbps, set some breakpoints, and let
’er rip. My program is written entirely in C++ using
the Borland compiler (large memory model).
A
lot’s been said about the poor suitability of C++
for real-time embedded development. But, it’s more
than adequate for a fast-response DeviceNet interface.
The
program weighs in at 45 KB of code space, including the
weather-station application code. This size is comparable
to DeviceNet interfaces I’ve done using standard
C with small CPUs. I’ll show you some performance
measurements later on.
|
