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C-1
Appendix C. REMOTE CONTROL
This section describes the protocol and message command set for remote monitor and
control of the CDM-QX Satellite Modem. The protocol is based on the CDM-570L,
but is modified to add the new features and to separately address the four plug-in
slots.
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.
C.1 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.