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User Interfaces
Note that multi-drop override IDs 01 or 02 can be used interchangeably to broadcast a message
to a DMD3000/4000 modem, or to a DMD4500/5000, or to a DMD15 modem. Radyne
ComStream Corporation recommends that the multi-drop override IDs be issued only during
system configuration as a bus test tool by experienced programmers, and that they not be
included in run-time software. It is also advantageous to consider the use of multiple bus systems
where warranted by a moderate to large equipment complement. Therefore, if a DMD2401 is
queried for its equipment type identifier, it will return a "27".
4.4.6 Software Compatibility
The DMD2401 RLLP is not software-compatible with the
following previous Radyne products:
DMD4500. These products may not occupy the same
bus while using this protocol as equipment malfunction
and loss of data may occur.
When Radyne equipment is queried for information (Query Mod, Query Demod, etc.) it responds
by sending back two blocks of data; a non-volatile section (parameters that can be modified by the
user) and a volatile section (status information). It also returns a count value that indicates how
large the non-volatile section is. This count is used by M&C developers to index into the start of
the volatile section.
When new features are added to Radyne equipment, the control parameters are appended to the
end of the non-volatile section, and status of the features, if any, are added at the end of the
volatile section. If a remote M&C queries two pieces of Radyne equipment with different software
revisions, they might respond with two different sized packets. The remote M&C MUST make use
of the non-volatile count value to index to the start of the volatile section. If the remote M&C is not
aware of the newly added features to the Radyne product, it should disregard the parameters at
the end of the non-volatile section and index to the start of the volatile section.
If packets are handled in this fashion, there will also be backward-compatibility between Radyne
equipment and M&C systems. Remote M&C systems need not be modified every time a feature
is added unless the user needs access to that feature.
4.4.7 Flow Control and Task Processing
The original packet sender (the M&C computer) relies on accurate timeout information with regard
to each piece of equipment under its control. This provides for efficient bus communication
without unnecessary handshake overhead timing. One critical value is designated the Inter-
Frame Space (FS). The Inter-Frame Space provides a period of time in which the packet receiver
and medium (control bus and M&C computer interface) fully recover from the packet
transmission/reception process and the receiver is ready to accept a new message. The
programmed value of the Inter-Frame Space should be greater than the sum of the "turnaround
time" and the round-trip (sender/receiver/bus) propagation time, including handshake overhead.
The term "turnaround time" refers to the amount of time required for a receiver to be re-enabled
and ready to receive a packet after having just received a packet. In flow control programming,
the Inter-Frame Space may be determined empirically in accord with the system configuration, or
calculated based on established maximum equipment task processing times.
4-42
DMD2401/DMD2401L/DMD2401 IBS/IDR Satellite Modem
RCU5000 and
TM065 – Rev. 3.3
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