The ENQ/ACK Protocol
ENQ/ACK is a flow control system invented by Hewlett-Packard in 1972. It was implemented in the HP 3000 line of midrange computers. XON/XOFF replaced ENQ/ACK in many later versions of the operating system.
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Function
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ENQ/ACK is part of the MPE-V operating system of earlier HP 3000 computers, and it is designed to aid communication between a computer and a terminal. The computer sends a special character after sending 80 characters. This is "ENQ." The terminal is expected to reply with an "ACK" character to signal it is ready to receive more data. The computer waits for 10 seconds if it does not receive an "ACK," then sends another 80 characters and another "ENQ."
Features
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"ENQ" and "ACK" are not words, but characters. The ASCII character set assigns a number to each character user in the English language. The first 32 characters in this set are non-printable characters used for communications. "ENQ" is character number 5 and means "Enquiry." "ACK" is character number 6 and means "Acknowledgment."
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Development
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More complicated communications use the ASCII character set for transmission control, although another character, "NAK," is usually included to show a "Negative Acknowledgment." "NAK" provokes the sender to send a packet again.
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References
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