Current working frequency of the transceiver can be extracted in two ways: parallel or serial. Only Yaesu transceivers provide parallel band data information. Similar information is provided by logging programs on the parallel port.
 
The band information is encoded into 4 bit BCD format: 1H is for 160m, 2H is for 80m etc. Decoding of this information is very fast and this method is often used by other band decoders. It is useful also when using cross band operation, where the band decoder can act upon band change immediately.
The drawback of this method is, that the band decoder does not know the frequency exactly, so it cannot switch devices within one band (e.g. matching ANT on 80m for CW and for SSB separately, etc.). Another drawback (when using LPT port of the computer) is, that the band decoder has no information of the transceiver frequency until the logging software not running. Hence it can not switch the appropriate antenna (or BPF, etc.) to the RIG. This applies also to (re)booting of computer.
 
Serial decoding is based on communication to the radio via its computer port. A big advantage of this method is, that the band decoder knows the actual frequency of the transceiver, and this enables to perform advanced functions such as to divide the band to more parts and to use a different output for each part, or to disable keying, if the transceiver is tuned out of a given range.
Another advantage of serial band data collection is, that the band decoder works with all types of transceivers, with or without a computer connected. If a computer is connected, the band decoder transparently forwards all commands of the logging program to the transceiver, while learning the working frequency online.
The only drawback of this method is a somewhat slower switching (reaction to band change delays of several hundreds of milliseconds), given by the speed of serial communication between decoder (computer) and radio.
 
 

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