Omega A2400 - Manual (Page 24)

Command Set 4-3
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
A communications time-out error can occur if the A2400 has not received a
command correctly. This may be due to noise on the communications
channel, incorrect address, hardware failures, etc. There is certain amount
of time after which it can be assumed by the host that a response message
will never occur. With the A2400, the response time is variable due to the
programmable delays built into the unit. Delay times T1 and T2 must be
added to the message times listed in Table 2.0 to calculate the maximum
time necessary to respond to a command.
Long Form Responses
When the pound sign ‘#’ command prompt is used, the module will respond
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 in
cases where the host wishes to verify the command received by the A2400.
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: *+99999.99
Command: #1RD (long form)
Response: *1RD+99999.99D9
Checksum
The 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 transmitted
to an addressable converter module as a user option. When a module