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If the station has two receivers it has the fully optionable backplane. There are three horizontal card slots above the verticle control cards and below the receivers and exciter. One of those is the R1 audio squelch, one is the R1 PL decoder & transmit PL encoder, the third slot contains the R2 audio squelch.
The R2 audio squelch contains its own PL decoder. There are three card versions: TRN9690 CSQ / TRN9691 PL / TRN9692 DPL.
You will find these in the 'Control and Audio' manual: 6881061E40