Thank you for answering.

Dave Platt wrote:

One*of*them*tells*the
radio that there's a slave CAT device attached, and the other tells it
to talk to a tuner via the CAT port.

When I select a tuner on FT-857 options, CAT type option turn automatically
to tuner and isn't changeable.

Since I try to connect and several options on this tuner, think I notice a
burned smell coming out from this tuner.
Works normally, maybe some device burned down by using any wrong option on
this MFJ-929.

73

Ralf, DL2MRB
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Vy 73 es 55 de Ralf, DL2MRB