5-2
Sequence Errors detected
Illegal Command detected
Base Station Initialized
Addressing a Terminal Not Signed In
Command without an ID
All of these error conditions are detailed in the next chapter. Don’t
forget to program for them; this is a common mistake. Failure to
trap them will give create very strange, unpredictable results.
Even though you don’t think your code will ever make a mistake, take
advantage of feedback that the Base Station provides. Failure to do so
is a common mistake that eventually results in serious program failure,
sometimes due to hardware problems that go undetected.
•
Parse the Returned Strings thoroughly
. Don’t assume anything
about the next response from the Base to your program and look
only for the partial string such as the ID only; parse the string
returned completely and be sure you are examining every
possibility. Failure to do so is a common mistake.
•
Plan for expansion
. You may start small (1 base/1 Terminal) but
try to create an application that will allow for easy expansion and
addition - especially of Terminals.
•
Use the Demo Programs
. The demo programs can at least allow
you to see how the system functions and whether you can
anticipate any system-wide problems. The demo programs should
also be used as a response-time benchmark.
Failure Planning
Hardware Failures
Let’s assume that each part of the system has failed. How are you going to
know what has happened and how are you going to recover?
•
The most frequent failures are at the Terminal level. If a Terminal has
a hardware failure, it will not be able to SIGN OUT. It is possible for
the Terminal operator to press the ON/OFF key or the F1 key by
accident, forcing the Terminal to SIGN OUT - sometimes in the
middle of a transaction. This happens at battery-changing time also.
You need to plan for partial transactions - do you trash the data you do
have and start over, or pick up where you left off?
•
Keep in mind that if a Terminal has SIGNED OUT in mid-
transaction, the Base Station clears any pending message for that
Terminal before it will allow it to SIGN ON again. Make
allowances to re-send messages or prompts that were cleared upon
SIGN ON if necessary.
