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A Hayes-compatible modem is a modem that recognizes and adheres to the Hayes AT command set, a command language with strings that combine together to output complete commands for different operations. Most modems follow the Hayes command set specifications to complete commands such as hanging up, dialing and changing connection parameters.
Modems that recognize the same set of commands defined by Hayes in the Hayes command set are called Hayes-compatible. This standard was developed for the Hayes Smartmodem in 1981. A string holds multiple Hayes commands placed together, which prepares the modem to dial out. Such strings are called initialization strings.
Initially, due to lack of proper written standards, other manufactures simply copied external visible commands and basic actions. Thus, there was a difference in how modems changed their states and their error-handling mechanisms. When required, manufacturers added new commands, making the modem more incompatible as some modems required spaces while others did not. In other instances, modem manufacturers changed baud rates, leaving computers clueless about how to handle incoming bits.
Hayes commands are set at beginning of every command line and are terminated with a CR (/r) character. Several Hayes commands can be used on the same line, eliminating the need to type AT before each command. Semicolons are used as command delimiters. If Hayes commands are to be entered on separate lines, a pause can be entered between the previous and the next command until an OK is encountered. This avoids having to send multiple Hayes commands at a time without waiting for every command response.