SCN 4138 was further improved upon to allow communication error retries to be non-destructive. In other words do no harm.

Communication errors occur in an industrial environment. If the response by the 6370 gets corrupted and the XL200 sends the command again, the retry must not do anything that would cause an error on the printer or the 6370.

The original 6370 commands all could be resent, with the exception of one, without doing any harm. For example, the 'T' or 't' commands could cause duplicate characters to be sent to a printer.

The new commands introduced in SCN 4138 are now all non-destructive, with the addition of a sequence number to the 'A' and 'a' commands. The sequence number is used by the 6370 to determine if a command is new or if it is a retry. If an 'A' or 'a' command has the same sequence number, the 6370 replies with the same response it replied with for the prior command, without actually doing anything because it already executed the command when it was sent originally.