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Chapter 11. REMOTE CONTROL
11.1 Introduction
This section describes the protocol and message command set for remote monitor and control of
the CDM-700 Modem.
The electrical interface is either an RS-485 multi-drop bus (for the control of many devices) or an
RS-232 connection (for the control of a single device), and data is transmitted in asynchronous
serial form, using ASCII characters. Control and status information is transmitted in packets, of
variable length, in accordance with the structure and protocol defined in later sections.
11.2 RS-485
For applications where multiple devices are to be monitored and controlled, a full-duplex (or
4-wire) RS-485 is preferred. Half-duplex (2-wire) RS-485 is possible, but is not preferred.
In full-duplex RS-485 communication there are two separate, isolated, independent, differential-
mode twisted pairs, each handling serial data in different directions. It is assumed that there is a
'controller' device (a PC or dumb terminal), which transmits data, in a broadcast mode, via one of
the pairs. Many 'target' devices are connected to this pair, which all simultaneously receive data
from the controller. The controller is the only device with a line-driver connected to this pair - the
target devices only have line-receivers connected.
In the other direction, on the other pair, each target has a tri-stateable line driver connected, and
the controller has a line-receiver connected. All the line drivers are held in high-impedance mode
until one – and only one – target transmits back to the controller.
Each target has a unique address, and each time the controller transmits, in a framed 'packet' of
data, the address of the intended recipient target is included. All of the targets receive the packet,
but only one – the intended – will reply. The target enables its output line driver, and transmits its
return data packet back to the controller, in the other direction, on the physically separate pair.
11–1
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