Z85233 Datasheet, PDF (266/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

APPLICATION NOTE
1
TECHNICAL CONSIDERATIONS WHEN IMPLEMENTING
LOCALTALK LINK ACCESS PROTOCOL
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T he LLAP Protocol is an important part of the Appletalk network system. It manages
access to the node-to-node transmission of network data packets, governs access to the
link, and provides a means for nodes to discover valid addresses...all error free.
INTRODUCTION
The LLAP (LocalTalk Link Access Protocol) is the ISO/OSI
(International Standards Organization/Open Systems
Interconnection) link layer protocol of the AppleTalk
network system. This protocol manages the node-to-node
transmission of data packets in the network. LLAP governs
access to the link and provides a means for nodes to
discover valid addresses. It does not guarantee packet
delivery; it does guarantee that those packets that are
delivered are error-free.
This Appnote (Application Note) does not address the
architectural issues of writing a driver but it does focus on
the details of using an SCC to send and receive LLAP
frames. However, some of the problems of transmitting
and receiving LLAP frames are discussed, using sample
code written for Zilog’s Z80181 Emulation Adapter Board.
Also, the problems of sending sync pulses, timing
transmissions and determining that a frame has been
received properly will be discussed.
GENERAL DESCRIPTION
The LocalTalk Link Access Protocol (LLAP) is the ISO-OSI
link layer protocol of the AppleTalk network system using
LocalTalk. Along with ELAP (the corresponding Ethernet
link layer protocol) and TLAP (the Token Ring link layer
protocol), it provides the foundations upon which the other
protocols rest. The LLAP protocol supports the node-to-
node transmission of packets used by DDP and RTMP to
route packets around the internetwork; DDP, in turn,
supports the name binding functions of NBP, the reliable
frame delivery of ATP, and the rest of the AppleTalk
protocol stack.
A majority of the difficult timing and all of the hardware
interface problems crop up in the LLAP driver. These
problems are so difficult that it makes sense to start writing
such a driver by writing experimental routines that transmit
and receive frames. This App Note addresses the
intricacies of the interframe and interdialog timings before
trying to engineer code that will truly be a driver. Also,
some of the experimental routines to run on the Z80181
Emulation Adapter Board will be explained.
The LLAP provides the basic transmission of packets from
one node to another on the same network. LLAP accepts
packets of data from clients residing on a particular node
and encapsulates that data into its proper LLAP data
packet. The encapsulation includes source and
destination addresses for proper delivery. LLAP ensures
that any damaged packet is discarded and rejected by the
destination node. The LLAP makes no effort to deliver
damaged packets.
Carrier Sense Multiple Access with Collision
Avoidance
It is LLAP’s responsibility to provide proper link access
management to ensure fair access to the link by all nodes
on that network. The access discipline that governs this is
known as Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA). A node wishing to gain access to
the link must first sense that the link is not in use by any
other node (carrier sense); if the link has activity, then the
node wishing to transmit must defer transmission. The
ability of LLAP to allow multiple access to the link also
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