Omega A2400 manual Long Form Responses, Command Checksum

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