enq

Enq, commonly written as ENQ, refers to the ASCII control character ENQUIRY. It is a non-printable code used in telecommunications and serial communications to request a response from another device or system. The character has the decimal value 5 and the hexadecimal value 0x05 (Unicode code point U+0005). As a control character, ENQ is not meant to produce visible output on most devices.

In operation, ENQ acts as a polling signal. A transmitting device sends ENQ to a remote station to inquire about its status or readiness. The receiving device is expected to reply, with the exact response depending on the protocol in use. Common responses can include an ACK (acknowledgement), a data frame, or an error signal, but there is no universal standard that governs every context. ENQ has been used in various legacy serial protocols, teletype networks, and early terminal communications, where simple handshake and status inquiries were common.

Modern computer networks and many current protocols rarely rely on ENQ, having largely replaced it with packet-based and higher-layer mechanisms. Nevertheless, ENQ remains a point of historical reference for understanding the evolution of remote polling and flow control in early computing and communications equipment.

In broader usage, enq is also a common shorthand for enqueue, the operation of inserting an element at the end of a queue in data structures and programming. In Unicode, ENQ is represented by the code point U+0005. See also ASCII control characters, ACK, and NAK.