Radio frequency dialog bus among three RMP’s.

After successfully putting together that one working RMP it was time to find another two for testing frequency dialog bus between all three RMP’s I was interested in. The remaining two RMP’s where supplied by APOC Aviation, one of my favorite aircraft part-out companies. I am sure you came across this company while searching for the original A320 parts. They have a really good website with current stock tab. Each part in stock is conveniently equipped with description, part number and a few pictures, making it easy for searching or just browsing.

Only a few days after the order, package arrived. Unpacking is always double sweet, with anticipated parts you also get typical Dutch “Stroopwafels”. The package will also include complete paperwork with part trace papers. After unpacking and crumbing wafels all over the workbench it was time to start testing.

I made all the connections according to pin-out diagram. I am lucky to have a genuine cannon plugs witch makes connections much easier. That is if you know which cable goes to which pin and searching for the right cable ends took me quite a while! I had to devote special attention to “location” SDI CODE cables and RMP cross talk dialog input/output ARINC cables. SDI CODE connections will determine RMP’s position in cockpit.

RMP1 or SEL1 is on Captain position, RMP SEL2 on First Officer position and  RMP SEL3 on overhead 3rd occupant/observer position. Each RMP will start with his dedicated (assigned) radio frequency RMP SEL1 starts with VHF1, RMP SEL2 with VHF2 and RMP SEL3 with VHF3-ACARS. There are also quite some ARINC cables for COM data, NAV data and dialog. You can recognize ARINC cables by light blue color and of greater thickness as cable is shielded around two data bus wires (A red and B blue).

Dialog bus output pins on all RMP’s are BB/CC unlike the input pins that swap between <x/<y and <z/AA.
– on RMP SEL1 <x/<y is SEL2 input and <z/AA SEL3 input,
– on RMP SEL2  <z/AA is SEL1 input and <x/<y SEL3 input,
– on RMP SEL3  <x/<y is SEL1 input and <z/AA SEL2 input.

The task of dialog bus is frequency exchange among RMP’s. Any new frequency selection or tune made on one RMP is taken into account by the others two. On one RMP you can select or tune any communication radio frequency for example when switching on RMP SEL1 from VHF1 to VHF2 the active VHF2 frequency should be the same as active VHF2 frequency on RMP SEL2 and similar for all other frequencies. If frequency selection and tune is done with unassigned RMP for example VHF2 frequency tuned on RMP SEL1 the active VHF2 frequency should also change on the assigned RMP SEL2 all thanks to dialog bus?

To avoid unintended selection or tuning of unassigned frequency the SEL-ect indicator lights will come on. SEL illuminates on both involved RMP’s if frequency selection is made on unassigned RMP.

Video of RMP dialog bus.

Radio frequency dialog bus among three RMP’s.
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