Page 243 - Embedded Microprocessor Systems Real World Design

Communication Pf'otoCOl Communication between processors can be imple-
mented with a proprietary mechanism such as those described earlier, or a stan-
dard protocol can be used. A typical example of a standard protocol is MODBUS.
MODBUS is a hardware-independent protocol that is used to communicate
between a master CPU and numerous slaves. A MODBUS data package sent from
master to slave includes the slave address (0 to 247), a function code, data (if
needed), and a checksum. The slave responds with a similar data packet to acknowl-
edge the transmission. MODBUS data can be transmitted as binary data, in which
case each transmitted data byte takes one byte to send, or as ASCII data, in which
case each transmitted data byte is sent as two ASCII bytes.
Since MODBUS is interface independent, you can use it to communicate over
RS232 or RS485 serial links, opencollector interfaces, or parallel port interfaces.
Using a standard interface protocol, even on a proprietary hardware interface,
provides some advantages. These include easier upgrades in the future and less
confusion during development of the software on the various processors.
For more complex systems, an Ethernet interface using TCP/IP or UDP (User
Datagram Protocol) can be used. This obviously requires a considerable step up in
complexity of both the software and hardware, so it is not well suited to a system
that must communicate with small microcontrollers.

Selection Criteria

When selecting a communication bus and protocol for a multiprocessor system, the
following factors should be considered:

Speed. Will the bus be able to keep up with your data rate? Be sure to take into
account polling in a master/slave architecture (see below).
Reliability. Do you need two redundant buses for high-reliability applications?
What about error checking? Can you assume that all commands are received cor-
rectly, or do you need a checksum on each block of data to prevent problems?
Does the hardware need protection against the possibility that the field engineer
will plug the interface cable into the wrong connector?
Standard/Proprietary. A standard bus, such as Ethernet, lets you buy off-the-shelf
cabling and use off-the-shelf boards and software, but it may be overly complex
for a simple system. In some products, the ability to plug a standard device in is
an advantage. In others, you want to keep your proprietary system proprietary.
OS Support. If you are using an off-the-shelf operating system, including an
RTOS, does it support the communication method and hardware you have
chosen? If not, you will have to write device drivers for it.
Bidirectional/Unidirectional. Sometimes a simple unidirectional interface is all
you need. Will you have problems if the requirements change or if system inte-
gration reveals the need for an interface in the opposite direction? You must be
sure no reverse path will be needed before choosing a unidirectional interface.

224 Embedded Microprocessor Systems