ACC HDLC/LAP-B (ABM) User's Guide

Chapter 3 37
Using HDLC/LAP-B (ABM) Protocol
Status and Error Messages
Unsolicited Status Messages
This section describes the unsolicited status messages which may be
received on the ZLU which is set as the receiver for a terminal using this
protocol. The status codes received may or may not have bit 7 set. Bit 7 is
the down bit (masked by
IO_DOWN_MASK
). The down bit is used as an
indicator of the link status. W hen an error occurs on the link or the link
is enabled after being disabled, the protocol attempts to enter multiple
frame acknowledged operation. Until the attempt succeeds or fails, the
down bit is clear (i.e. the terminal is considered to be up until proven
otherwise). If the attempt subsequent ly fails, the down bit is set;
otherwise the down bit is left cleared.
The following status messages are generated in response to terminal
enable and disable requests:
0 IO_OK No error detected
This status message is generated in response to a
terminal enable request after multiple frame
acknowledged operation is established. Note that the
down bit will always be clear (bit 7 = 0).
41 IO_ALRDY_ENBL Terminal already enabled
This status message is generated in response to a
terminal enable request when the terminal is already
enabled and the protocol is not currently trying to
establish multiple frame acknowledged operation. If
the protocol is currently trying to establish multiple
frame acknowledged operation, this status message
will not be sent. Instead, either a
ST25ENBL
with the
down bit set or an
IO_OK
status message will be sent.
42 IO_ALRDY_DSBL Terminalalready disabled
This status message is generated in response to a
terminal disable request when the terminal is already
disabled. The down bit will always be set.
64 ST25ENBL Link NOT established on ENABLE
This status message is generated in response to a
terminal enable request after it is determined that
multiple frame acknowledged operation can not be
established. The down bit will always be set.