Page 114 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Communications and Control

The template is then requested by the host and the slave assembles the scattered scraps of
data into a single message block. The efficiency of this messaging system is incred-
ible, and it is easily implemented.

Post offices

In complex systems such as mobile robots, communications patterns shift with the
nature of the task at hand. Furthermore, communications channels are shared resources
that many tasks (clients) may need to use. In the case of a base station program,
communications may be dictated by the number and type of forms that the operator
has open. It is therefore intolerably inefficient for a particular thread to wait in a
loop for the channel to become available, and then for the data to arrive.
In an efficient architecture, a task will request data before it absolutely needs it, and
while it can still do useful work before the data arrives. To do resource sharing on a
case by case basis rapidly results in unreadable code. Thus, we come to the general
concept of a resource manager.
A resource manager for a communication channel is usually called a post office. A post
office is a module (or better yet an object) that contains an array of pointers to
messages and their associated flags, and destination addresses. The elements of this
array are called post office boxes. At a minimum, a post office will have the following
methods available for the clients to use:
1. Post a request message

2. Post a send message
3. Provide status of a posted message

4. Provide status of the communications resource.
As a task needs to communicate with other systems, it calls the post office and posts
its message. The post office puts the message in the next available box, and then re-
turns a message identification code that the client can use to request the status of its
message in the future.
The reason that the post office does not simply return the box number is that boxes
are reused as time goes on. If a client does not pick up its message information before
the post office reuses the box, then the data in that box may have nothing to do
with the client’s request. In some schemes, the message identifier contains the box
number in its lower bits to make it easy for the post office to locate the message
when requested to do so by a client.

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