What can I connect to the RCA TX conenctor?

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I wanted to build a simple relay circuit that switches off the speaker from a power amp during transmit.  I read the specs in the hardware manual for the RCA TX  ports on the back of the Flex:

Δ – The TX Relay outputs are designed to handle signaling levels of up to +40 VDC @ 140 mA maximum.  Some amplifiers do not have circuits to prevent keying voltage transients and older amplifiers may exceed the maximum voltage level resulting in damage to the radio if directly connected.  Verify the voltage/current on your external device before connecting to TX Relay 1-3.  FlexRadio highly recommends the use of a use a buffer/isolator box between the radio and the external device as a best operating practice regardless of the keying voltage

I have a small 12V relay that draws about 13mA (both measured and calculated).  My question is:  What is driving the TX output?  Is there a relay internal in the Flex or is it some other type of solid state device?  I measured about 3.5V at the hot side of the TX port when transmit was not engaged and it clamped down to 0V during transmit.   Is it safe to run 12V through there to engage my relay  during transmit ?   Seems like it should be... but  I'd rather ask before hooking anything like that up.   Shouldn't it operate as a simple contact closure during transmit ?
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Kevin LaFata / K0KEV

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Posted 2 years ago

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Bob Wright, N7ZO

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Official Response
Gerald, K5SDR, said it was a TPIC6B595 driver and gave this link:
www.ti.com/lit/ds/symlink/tpic6b595.pdf

Graham, KE9H, gave this info:
"Open circuit voltage should not exceed +40 Volts DC. Closed circuit
current is 150 mA continuous max, and 500 mA peak max. There is the
equivalent of a +45 Volt Zener across the output to ground that will clamp
any positive going transient peaks at about +45 to 50 Volts. "

73, Bob, N7ZO
(Edited)