Comtech EF Data DD2401 VME - Manual (Page 35)

DD2401 VME L-Band Demodulator Card Installation & Operational Manual User Interfaces
MN-VME2401 – Rev. B 4-11
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 DD2401/DD2401L is queried for its equipment
type identifier, it will return a “27”.
4.4.6 Software Compatibility
The DD2401/DD2401L RLLP is not software-compatible with the following
previous Radyne products: RCU5000 and 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.
Each piece of supported equipment on the control bus executes a Radyne Link Level Task
(RLLT) in accordance with its internal hardware and fixed program structure. In a flow control
example, the RLLT issues an internal “message in” system call to invoke an I/0 wait condition that
persists until the task receives a command from the M&C computer. The RLLT has the option of
setting a timeout on the incoming message. Thus, if the equipment does not receive an
information/command packet within a given time period, the associated RLLT exits the I/0 wait
state and takes appropriate action.