© 2000 Ubicom, Inc. All rights reserved. - 11 - www.ubicom.com SX IrDA Virtual Peripheral Implementation AN16 3.3.7  Implementation - Payload Layer Receive For incoming frames the payload layer tests the connec-
tion address, and if it is correct it will inform the LAP layer
of the type of the frame. The information field will be pro-
cessed or passed (in the case of a data frame) as appro-
priate  for  the  type  of  frame,  and  a  Valid/Error  message
will be sent to the LAP layer when complete. When  the  LAP  layer  is  informed  of  the  incoming  frame
type   it   may   choose   to   ignore   the   frame   by   calling
lap2plRxIgnore  which  will  cause  the  payload  layer  to
ignore the frame.
Figure 3-5 shows the Payload Layer Receive API. If the lap layer calls lap2plRxIgnore to ignore the frame
then the payload layer will inform the framing layer that
the  frame  should  be  ignored  thus  no  more  data  will  be
received from the framing layer and no Valid/Error indica-
tion will be given.
The command byte is recorded by the payload layer so
that the LAP layer can examine the it once a frame has
been   completed.   This   allows   the   LAP   layer   to   gain
access to the frame numbering bits encoded in the com-
mand byte.
For  I  (reliable  data)  and  UI  (unreliable  data)  frames  all
data is passed to the LAP layer using pl2lapRxIData or
pl2lapRxUIData.
For  XID  frames  the  frame  information  field  is  internally
processed  by  the  payload  layer  except  for  the  XID  dis-
covery information field which is passed to the LAP layer
using   pl2lapRxXIDData.   The   received   32-bit   source
address is stored so that it can be used in a reply if the
frame  is  an  XID  command  frame  and  the  LAP  layer
requests that an XID response be sent. The received 32-
bit  destination  address  is  tested  to  ensure  it  is  either
broadcast  or  specifically  destined  for  the  IrDA  Virtual
Peripheral, and the XID flags and slot number are stored
for future reference by the LAP layer. If there is any error
in the frame such as a wrong destination address or an
unsupported  version  then  the  lap  layer  will  receive  a
pl2lapRxError message. Note that the payload layer only keeps  the  current  source  address  in  memory  and  does
not generate a list of received source addresses.
For SNRM frames the information field is internally pro-
cessed by the payload layer. The received 32-bit source
address  is  stored  so  that  it  can  be  used  in  a  reply  if
requested by the LAP layer. The received 32-bit destina-
tion address is tested to ensure it is either broadcast or
specifically destined for the IrDA Virtual Peripheral, and if
not then the lap layer will receive a pl2lapRxError indica-
tion. The received connection address is stored but not
applied (it cannot be applied until the frame has been val-
idated and the LAP layer requests that the connection be
accepted).   And   finally   the   baud   rate   parameter   is
extracted    from    the    received    supported    connection
parameters and stored for future use. When the frame is
validated  the  LAP  layer  will  accept  the  connection  and
the  stored  data  will  be  used  to  form  the  connection-
accept reply.
For all other frame types the information field is ignored. 3.3.8  Implementation - Payload Layer Transmit The payload layer provides the LAP layer with the ability
to request that a frame of a given type be sent.
When the LAP layer requests that a frame be sent it will
be sent immediately and the receive state will be reset to
idle. pl2lapRxFrame (w=Type) A correctly addressed frame of the given type is being received. pl2lapRxValid () The frame is complete and has been validated. pl2lapRxError () The frame is invalid. pl2lapRxXIDData (w=Data) Data from the XID information field pl2lapRxIData (w=Data) Data from an I frame pl2lapRxUIData (w=Data) Data from an UI frame lap2plRxIgnore () The lap layer has no interest in the frame. Figure 3-5. Payload Layer Receive API