IBM Support

Bisync DLC Character Codes in Communications Trace on OS/400 or i5/OS System

Technote (troubleshooting)


This document provides a listing of bisynchronous control characters for aid in translating the output of a communications trace.

Resolving the problem

The following is a list of Data Link Control Characters (also called transmission control characters) used in binary synchronous (Bisync or BSC) communications and a brief description of their use.

Character Hex (as seen in trace) Use
ACK0 / ACK1 (Alternating positive acknowledgements) 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).

Historical Number


Document information

More support for: IBM i

Software version: Version Independent

Operating system(s): IBM i

Reference #: N1017117

Modified date: 09 February 2013