
1–7Network Layers
Publication
17706.5.16 - October 1996
Your DF1 links and network links (DH, DH+, and DH485) each use
two layers of software to enable communication:
• the data-link layer
• the application layer
This figure shows how these layers fit together.
RS232
data link
layer
common
application
routines
application
computer
DH/DH+/DH485
RS232 interface modules
PLC processor
application
common
application
routines
DH/DH+
/DH485 data
link layer
link layer
(network)
link layer
(DF1)
RS232 DH/DH+
DH485
Datalink Layer
This layer controls the flow of communication over the physical link
and:
• determines the encoding of data on the physical medium
• controls who transmits data and who listens using an
arbitration protocol
• conveys data packets intact from the source node to the
destination node over the physical link
If your link is Then
a DH, DH+ or DH485 link
you do not need to program this layer.
Your application programs on the network
are not involved with internode protocol,
handshaking, or control of the link.
a DF1 link between AllenBradley
interface modules
the interface modules automatically take
care of this layer. Your application
program does not have to be involved with
handshaking control.
a DF1 link between an AllenBradley
interface module and a computer
you must program this layer at your
computer.
Programming the data-link layer
You can program the data link layer using DF1 protocol.
For more information, see Chapter 3, “Using Half-duplex Protocol to
Send and Receive Messages,” or Chapter 4, “Using Full-duplex
Protocol to Send and Receive Messages.”
Software Layers
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