2.2. Host and Target Examples
To simplify the protocol, the link can only comprise one host and one target. The Host is the only one that can set or
request Control items. This means a Target device cannot connect to another target device or daisy chain to other
targets. The Host can control multiple Targets by utilizing multiple links such as USB endpoints or multiple TCP/IP
Sockets.
Target
Host
Target1
Host
Target2
Target1 Target2
Host1 Host2
Application Program
Link2Link1
The protocol allows a Target to send unsolicited Control Item messages to the Host. This is desirable for updating
the Host when a something changes in the Target without the need for polling by the Host. An example would be when a
user changes the frequency of the radio using the radio's frequency knob. The target can send the updated frequency as
it occurs without requiring the Host to ask for it.
Message blocks contain the block length in the message header. This is useful to aid in decoding messages as well
as being able to support variable length Control Items. For example, a Control Item containing the text string for the
Target's manufacturer and model number can be different lengths.
Target devices are not required to implement all the functionality of the protocol. Any unimplemented message will
return a NAK response.
The Data Item message blocks allow various raw data blocks to be sent and received along with the Control Items
over the same physical link or separate physical links. The Header type allows up to 4 logical channels of data to be
specified in each direction. This permits sending digitized audio, digitized I/Q IF data, etc. to and from a target over the
same physical connection.
Note that there is no synchronization or error handling mechanism in this protocol. This layer of protocol assumes
that the block synchronization and error handling is done at a lower level. This is a reasonable assumption since
Ethernet, USB, IEEE 1394, and most other modern physical links provide error recovery.
Rev. 1.03 2011-11-01
8
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