Envoy ACP/XF Application Programming Manual

Asynchronous Balanced Mode (ABM) Protocols
EnvoyACP/XF Application Programming Manual–132179
2-2
ABM Link-Level Commands
Figure 2-1 shows station addressing for combined stations. The two stations each
maintain a link-level send (Ns) and receive (Nr) sequence number to ensure the proper
sequencing of information frames.
ABM Link-Level Commands
EnvoyACP/XF transmits the following link-level commands from the local station to the
remote station as the result of explicit application-program calls to the CONTROL file-
management procedure:
Contention Resolution
When a station issues a mode-setting command (SABM, SABME, or DISC), the station
should receive a response from the remote station; if the sending station receives a
mode-setting command instead of a response, the two stations are contending for control
of the line. If the two mode-setting commands are the same, each station sends a UA
response at the first opportunity. If the two mode-setting commands are different, each
station sends a DM response and disconnects.
Figure 2-1. Station Addressing
SABM Set Asynchronous Balanced Mode
SABME Set Asynchronous Balanced Mode Extended (extended mode only)
RSET Reset local Ns and remote Nr frame-sequencing variables to 0
DISC Set Asynchronous Disconnect Mode (ADM)
011
Primary/Combined
Station
(For Command and
I-Frame from Local Station)
Address 1
(For Command and I-Frame
from Remote Station)
Address 2
Local Combined
Station
Remote Combined
Station
(For Response from
Remote Station)
(For Response Frames
from Local Station)
Secondary/
Combined Station
Primary/Combined
Station
Secondary/
Combined Station