Specifications

Command Set 4-4
following the ‘ * ‘ prompt. The response format of all commands may be found
in the detailed command description.
The maximum response message length is 20 characters.
A command/response sequence is not complete until a valid response is
received. The host may not initiate a new command until the response from
a previous command is complete. Failure to observe this rule will result in
communications collisions. A valid response can be in one of three forms:
1) a normal response indicated by a ‘ * ‘ prompt
2) an error message indicated by a ‘ ? ‘ prompt
3) a communications time-out error
When a module receives a valid command, it must interpret the command,
perform the desired function, and the communicate the response back to the
host. Each command has an associated delay time in which the module is
busy calculating the response. If the host does not receive a response in an
appropriate amount of time specified in Table 4.1, a communications time-
out error has occurred. After the communications time-out it is assumed that
no response data is forthcoming. This error usually results when an
improper command prompt or address is transmitted.
Long Form Responses
When the pound sign ‘ # ‘ command prompt is used, the module responds
with a ‘long form’ response. This type of response will echo the command
message, supply the necessary response data and will add a two-character
checksum to the end of the message. Long form responses are used when
the host wishes to verify the command received by the module. The
checksum is included to verify the integrity of the response data. The ‘ # ‘
command prompt may be used with any command. For example:
Command: $1RD (short form)
Response: *+00072.10
Command: #1RD (long form)
Response: *1RD+00072.10A4 (A4=checksum)
Checksum
Checksum is a two character hexadecimal value appended to the end of a
message. It verifies that the message received is exactly the same as the
message sent. The checksum ensures the integrity of the information
communicated.
Command Checksum
A two-character checksum may be appended to any command to the
module as a user option. When a module interprets a command, it looks for