3.1. Detailed Description of the Message Block Types and Their Purpose
3.1.1 Set Control Item
This Message type is sent from the Host to the Target requesting that the Target change the specified Control Item to
the new value supplied in this message. A request to change to a new frequency would be an example of this type of
message. The Target must respond to this message either with a NAK or it returns the response message(just a copy of
the received Set message).
3.1.2 Request Control Item
This Message type is sent from the Host to the Target requesting that the Target respond with its current state or
value of the specified Control Item of this message. A request to get the current S-meter reading would be an example of
this message type. The Target must respond to this message either with a NAK or it returns the current requested control
item value(s).
3.1.3 Request Control Item Range
This Message type is sent from the Host to the Target requesting that the Target respond with the acceptable range
of values of the Control Item supplied in this message. A request for the targets frequency range(s) and step sizes would
be an example of this message type. The Target must respond to this message either with a NAK or it returns the current
control item range(s). (The NetSDR implements this message type on a few select Contol Items.)
3.1.4 Response to Set or Request Current Control Item
This Message type is sent from the Target to the Host in response to a request from the Host to either set or just
return the current value of the Control Item supplied in this message. This message contains the current value of the
Control Item. It is sent in response to either the "Set Control Item" or "Request Control Item" message.
3.1.5 Unsolicited Control Item
This Message type can be sent from the Target to the Host without any request from the Host. It contains the current
value of the Control Item supplied in this message. This message can be sent at any time to the Host. It can be used to
update the Host to any changes that have occurred in the Target Control Items. An example would be if the user changed
frequency using the Targets frequency knob, then the Target could send the new Control Item value to the Host without
having to wait for the Host to ask for it. There is no response back from the Host when this message is received.
3.1.6 Response to Request Control Item Range
This Message is sent from the Target to the Host in response to a "Request Control Item Range" message from the
Host. It contains the allowable range and step size of the Control Item supplied in this message.(The SDR-xx does not
implement this message type.)
3.1.7 Data Item Messages
Data Item message allow data messages to be allocated to different logical "channels". Different types of data
blocks may be interleaved together and this mechanism allows each end to keep the data separated. For example, Data
Item 2 blocks may be digitized audio from a Target receiver that needs to be processed and sent to a soundcard speaker.
Data Item 3 Blocks may be spectral data from an FFT inside the Target receiver that needs to be sent to the Host
applications display screen.
The current scheme allows up to four different logical channels for each data direction.
Rev. 1.03 2011-11-01
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