| Character | Hex | Use |
|---|---|---|
| ACK0 / ACK1 (Alternating positive ACKs) | 1070 / 1061 | Indicates that text was received without errors. Also is a positive response (acknowledgement) to an ENQ (bid) on a point-to-point line. In 3780 Bisynchronous Communications, this positive acknowledgement alternates between 1070 (even) and 1061(odd). |
| DISC (Disconnect) | 1037 | Indicates a mandatory disconnect on a switched line connection. The station sending this message will disconnect without having to receive confirmation from the receiving station. |
| DLE (Data Link Escape) | 10 | Changes the meaning of the control character immediately following the DLE. For example, the sequence DLE EOT indicates that both stations on a switched connection should disconnect from the line. |
| ENQ (Enquiry) | 2D | Has the following uses:
-- Initiates (bids for) control of transmission on a point-to-point connection -- Indicates the end of a polling or address sequence on a multi-point connection -- Indicates a request for a station to repeat its response (received from target station) -- Indicates that a block of text should be ignored (if the blocks ends with a ENQ) -- Indicates the end of an identification sequence on a switched connection |
| EOT (End Of Transmission) | 37 | Indicates the end of a transmission for the sending system. This is also sent when the sending system exhausts its retry limit of sending an ENQ to a remote system without a response. |
| ETB (End Of Text Block) | 26 | Indicates the end of a block of data, but not the last block transmitted (more data will be coming from the station that transmits this). |
| ETX (End Of Text) | 03 | Indicates the end of a message if ETBs are used (text is transmitted in blocks). Also can indicate the end of the last block of text transmitted (completed text transmission) |
| ITB (Intermediate Text Block) | 1F | Divides a block of text into smaller groups of text (Intermediate Text Blocks) for error checking. |
| NAK | 3D | Indicates a Not Ready condition (most common), or indicates that an error occurred. Also is sent as a negative response to a line bid or switched line security identification. |
| Pad (Leading Pad Character) | 55 or AA | At least 2 leading pad characters are transmitted to synchronize the clocks at the transmitting and receiving stations. |
| Pad (Trailing Pad Character) | FF | A trailing pad character ensures that all bits of the last character of a message are transmitted. |
| RVI (Reverse Interrupt) | 107C | Transmitted by the receiving station rather than an ACK to indicate a positive acknowledgement and to request that the transmitting station stop transmitting so that the receiving station can transmit. (some people may refer to this as a "turnaround request") |
| SOH (Start Of Header) | 01 | Indicates the start of heading information. |
| STX (Start Of Text) | 02 | Indicates the start of text. |
| SYN (Sync) | 32 | Establishes and maintains character synchronization. |
| TTD (Temporary Text Delay) | 022D | Indicates to the receiving station that the transmitting station is temporarily not ready to transmit. (station that transmits this is having an issue) |
| WACK (Wait Before Transmitting) | 106B | Transmitted by the receiving station to indicate to the transmitting station a positive acknowledgement and a temporary not-ready-to-receive condition (this is usually recovered by the application at which point the transmission continues). |