Allen-Bradley 1779-KFMR User Manual page 16

A dumb terminal can only communicate across a Data Highway II
network through the PAD protocol because the terminal does not have the
intelligence to assemble or disassemble message packets. When a
computer sends a message across the network to a terminal node, at the
receiving node, the interface disassembles the packet and sends only the
data field to the terminal. Furthermore, by issuing a station-management
command from another station, you can establish a logical connection
from the terminal to the computer so that if the operator at the terminal
enters a data field, the interface assembles the data field into the proper
packet for the network to deliver the message to the computer.
The PAD protocol is also useful for devices, such as robots, which have a
fixed protocol. In sending a message to a robot, a computer could encode
the message data field with the protocol that the robot expects. At the
receiving node, the interface disassembles the packet and sends only the
data field to the robot. Since the computer encodes the data field, the
robot sees the protocol that it expects.
Data Highway II Link
In a Data Highway II link, nodes communicate with one another through
coaxial cable which provides a physical bus medium. The coaxial cable
forms the bus connection between the nodes in a trunkline/dropline
configuration.
The maximum length permissible for the trunkline depends on the number
of nodes, the type of cable, and the type of tap (local tap or extended tap).
For example, with RG-11-type coaxial cable approved by Allen-Bradley,
local taps, and 52 nodes in a link, the trunkline length can be up to
4,100 cable-feet. Refer to Chapter 3 for a list of approved cables and
other detailed information on planning cable lengths.
The Data Highway II link provides carrier-band signaling through
frequency shift keying. The carrier frequency is 5 MHz. The carrier is
modulated to 6.25 MHz for a data-high signal and 3.75 MHz for a
data-low signal. Because it has a carrier-band medium, the Data
Highway II link provides high noise immunity at its high data
transmission rate (1 Mbit/s) over its long permissible cable lengths.
Chapter 2
Introducing the 1779 KFM Interface
2 9