Envoy ACP/XF Application Programming Manual

Normal Response Mode (NRM) Protocols
EnvoyACP/XF Application Programming Manual–132179
3-18
NRM Basic Message Transmission
reserve WRITEREAD requests for special functions, such as sending U-frames (that is,
U-frames for which you must specify the MCW rather than issuing a CONTROL call).
Do not use WRITEREAD requests routinely for sending I-frames.
SETMODE 30 allows nowait I/O operations to complete on the application level in the
order of their completion by the I/O process, rather than in the order in which they were
issued.
NRM Basic Message Transmission
A message is the data string within the information field of an I-frame or U-frame. In an
EnvoyACP/XF SDLC or ADCCP environment, each WRITE call (or write portion of a
WRITEREAD call) transmits a message and each READ call (or read portion of a
WRITEREAD call) accepts a message, as in the following example:
CALL READ <——————————————————————|
F A C message FCS F
F = flag
A = address field
C = control field
message = information field
FCS = frame check sequence
A transmission is a logical set of messages transferred over the line between P/F bits that
are set (=1), as in the following example:
CALL READ <———————————————————|
(CCE) F A C(P/F=0) message FCS F\
\
CALL READ <———————————————————| |
(CCE) F A C(P/F=0) message FCS F |
. . |= transmission
. . |
|
CALL READ <———————————————————| /
(CCG) F A C(P/F=1) message FCS F/
EnvoyACP/XF treats a WRITEREAD request as two requests: a WRITE request
followed by a READ request. EnvoyACP/XF does not recognize the read portion of the
WRITEREAD until the write portion of the WRITEREAD completes. If the application
issues a separate READ request after a WRITEREAD but before the write portion of the
WRITEREAD completes, the separate READ completes before the WRITEREAD:
CALL READ <———————————————————|
(CCG) F A C(P/F=1) message FCS F
CALL WRITE |———————————————————>
(CCE) F A C(P/F=0) message FCS F
CALL WRITE |———————————————————>
(CCE) F A C(P/F=0) message FCS F
CALL WRITEREAD |———————————————————>
F A C(P/F=1) message FCS F
F A C(P/F=0) message FCS F
(CCE) <———————————————————|