User's Manual

Page 44
M Series Data Radio – User Manual
© Copyright 2004 Trio DataCom Pty. Ltd.
Part H – TVIEW+ Management Suite - Programmer
Packet Layer
There are two standard configurations and a custom configuration
which can be selected by checking the appropriate control button to the
left of the description. There are essentially two basic modes of
operation for the packet assembler and disassembler (PAD).
The first is where the PAD operates in a standard mode with data
received at the port being immediately sent over the radio channel.
The second is a store and forward or delayed mode where whole data
packets are received from the port before being sent over the radio
channel.
In both cases data is sent over the radio channel in variable length
frames and delineation of these frames is dependent on the
configuration selected as well as the characteristics of the data stream
received at the data port.
The packet layer configuration options which can be selected are:
Standard (live framing)
With standard live framing data received from the host by the modem
is immediately placed into a frame and transferred onto the radio
channel.
This avoids placing “store and forward” delays in the data
transmission.
If a stream of characters is received by the modem, then several
characters at a time may be placed into the same frame. The number
of characters in the frame depends mainly on the respective baud
rates of the user port and the primary channel baud rate of the modem,
as well as the level of overheads experienced on the radio channel
and the user data stream.
For example a constant stream of 300 baud user data placed onto a
9600 baud channel will result in 1 character per frame being
transmitted. If the user baud rate was lifted to 9600,N,8,1 with a
continuous data stream, then the frame size would settle to about 16
characters plus 32 overhead bits. If collision avoidance is enabled as
master the average frame size will increase to 32 characters plus
overhead bits.
The number of data bits associated with the user data stream will also
have an effect on the average size of a frame. For instance the
number of stop bits, and number of data bits per character.
The system designer must choose the best compromise of all the
above items to ensure the most efficient method of data transmission.
Note: The first character is always packetised and sent by itself
regardless of all the above variables.
Modbus
This selection configures the PAD driver with options automatically set
to implement the MODBUS protocol, e.g. 5 mSec timer.
Custom
Other configurations of the PAD driver can be selected via the Custom
button which displays a dialogue box to permit selection of several
configuration options as follows:
SLIP / DIAGNOSTICS
SLIP is a well known protocol for transferring binary data
packets over a data link. Each data packet is delineated by
<FEND> characters, and a substitution mechanism exists that
allows these characters to be included in the data packet.
Appendix B describes the SLIP protocol which is used
extensively in UNIX™ based systems, and is closely
associated with TCP/IP networks.
The diagnostics controller package uses the SLIP protocol to
communicate between base station and remote modems.
DNP-3 / IEC870
This selection configures the PAD driver to implement the DNP-3
Protocol and IEC870 Protocol.
Pull Down Menu Selection
The PAD driver can be configured for a number of vendor
specific protocols by selecting the desired option.
Custom Format
This selection permits PAD driver to be configured in a variety of
ways and requires a greater understanding of the system design.
For the modem to successfully transmit its packets (or frames) of
data over the radio channel, it must be told on what basis to
delineate data packets received at the data port. Once the end of
a data packet has been received at the port the data frame is
closed and transmission over the radio channel commences.
Delineation of data packets can be configured to occur via any
combination of: