Envoy ACP/XF Application Programming Manual

Asynchronous Balanced Mode (ABM) Protocols
EnvoyACP/XF Application Programming Manual–132179
2-6
Extended Address Field
should ensure that the station addresses for the local and remote computer systems are
configured compatibly.
Extended Address Field
You can use SCF to change the ADDRESSSIZE modifier to enable the extended address
format or you can use the SETMODE 116 procedure call. If the address size is no more
than 2-octets, you can use SETMODE 16 to specify the primary/combined station and
secondary/combined station. To specify an extended address longer than 2-octets, you
must use the DEFINELIST procedure call.
Extended Control Field
By using the SETMODE and CONTROL procedures, you can configure a line to use
either a 1-octet or 2-octet control field.
Using SCF, you can specify a 2-octet control field by specifying the modifier
EXTENDEDCONTROL. Set a 1-octet or 2-octet control field by issuing a SETMODE
116 call before issuing any CONTROL, READ, WRITE, or WRITEREAD calls. Bit 3
of parameter 2 in the SETMODE 116 call specifies the control-field length, as follows:
CALL SETMODE (fnum,116,parameter1,parameter2)
where parameter2.<3> = 0: 1-octet control field
1: 2-octet control field
To initialize an ABM line, an application for a primary/combined station directs
EnvoyACP/XF to transmit the SABM or SABME command as follows:
CALL CONTROL (fnum,13,2);
When you use an extended control field, make sure that you have specified an
appropriate window size. The window size determines the number of I-frames that the
local station can send to a remote station before requiring acknowledgment. For basic
control modes, this value must be in the range 1 through 7; for extended control modes
the value may be in the range 1 through 127.
You can use SCF to specify window size by using the L2WINDOW modifier. You also
can specify the window size dynamically through a SETMODE 18 call that states the
value in parameter 2.
Control
Field
Frame
Sequence
Numbers Modulo Description
1-octet 3 bits modulo-8 Can transmit up to 7 I-frames in succession without
receiving an acknowledgment from the remote
station
2-octet 7 bits modulo-128 Can transmit up to 127 I-frames in succession
without receiving an acknowledgment from the
remote station
Note. Unless you specify a different value, the window size defaults to 7, no matter what the
length of the control field.