What can I connect to the RCA TX conenctor?

Kevin LaFata

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 ?

Paul Christensen, W9AC

Kevin,

Here's an instance where schematics from Flex would be useful.  I understand the need to protect intellectual property but at the very least, let us see I/O schematics.  How about adding simple images to the SSDR manual that show the basic I/O circuits, including the mic preamp stage?     

As I recall, either open collector NPN transistors or open drain FETs are used on the TX OUT lines.  These are used as switching transistors, just like a SPST pushbutton switch.   When you measured +3.5VDC on the RCA connector with the line deactivated and 0V when active, that strongly suggests  a pull-up resistor is used to a +Vcc logic supply bus. 

That's  surprising to me because most equipment that use switching transistors do not use a pull-up resistor unless there's a need to have a positive voltage present in the OFF state. Possibly Flex wanted to have two active logic states at the RCA connector. If that's the case, hopefully Flex included a steering diode in series with that pull-up resistor so that Flex's internal supply and the external supply used for your relay are adequately isolated from one another. Possibly someone from Flex can describe the exact circuitry from the TX OUT jacks.  

You can use the TX OUT jack as intended and connect it to one side of your +12V relay.  Connect the other side of the relay to your +12V station supply or wall wart.  I strongly suggest using: (1) an in-line fuse of 100mA to protect the Flex TX OUT line; and (2) a 1N914 steering diode in series with your relay (anode toward your +12v supply).  I am using optical isolators for my amp switching and there's not been any type of problem if indeed a pull-up resistor is used. 

Here's what's happening when using a switching transistor to pull-in the relay: one side of the relay is connected to your external +12V supply.  The other side is connected to the switching transistor at the TX OUT jack.  When enabled in SSDR, the switching transistor closes the circuit path to ground on transmit.  Think of the transistor as acting just like a SPST switch. 

At this point, other folks will want to jump in and advocate the use of a keying buffer circuit (and as recommended by Flex).  That's normally unnecessary in your application when you know the relay current is 12 mA and the maximum voltage on the key line is +12V --  both of which are way under Flex's TX OUT key line rating.  Indeed, an external relay IS a circuit buffer.  I say "normally" because we really need a better understanding of Flex circuit topology. 

Finally, we know that hundreds of Flex transceivers are connected to external amplifiers and aux devices that use either solid-state or relay key line switching.  But just because we know it works doesn't mean we should not understand Flex's driving circuitry.   

Paul, W9AC

    

W5XZ - dan

a LONG time ago, gerald showed us the chip that drives the rca tx connectors...but, i'll be darned if I can locate it now..

Paul Christensen, W9AC

Possibly an array driver chip like this...

http://www.ti.com/product/ULN2003A

Paul, W9AC

W5XZ - dan

found it..

https://community.flexradio.com/flexradio/topics/what_are_the_specifications_for_the_tx_relay_output...

Bob N7ZO

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

Paul Christensen, W9AC

Bob,

Thanks for confirming.  So, getting back to Kevin's post...

He can use his 12V relay with one side connected to TX RLY CTL and the other side to his external +12V power supply.  No need for an external buffer circuit as the relay is a buffer in this application.  I still recommend a 100 mA fuse in the relay coil lead. 

Paul, W9AC

Kevin LaFata

Thanks everyone, this is all great info!   Definitely doesn't seem like a simple "contact closure", and I think if I connected 12VDC to the shield side of the TX port it would most definitely short, so it does seem that one would have to be very careful in designing what is connected where. 

Bob N7ZO

Paul and all,

It would still be good practice to put a flyback diode across the relay.  But be aware that a flyback diode can slightly delay the relay's release (normally not a problem).

73, Bob, N7ZO

Paul Christensen, W9AC

Bob,

The TPIC6B595 includes an internal snubber diode.  Also, there's just not much EMF back-snap from the collapsing field of a 12mA/12V relay to warrant an additional diode across the coil.

Paul, W9AC

Dick B

If it's open collector/drain there should be no problem connecting the Flex TX line in parallel with a real relay in my ICOM so they can both key my solid state amp.  True?  (Been doing it for months.  Not at the same time of course. )  

I was pretty certain that I checked my prior Flex (a 3000) and it used a relay, so I didn't even think to check when I replaced the 3000 with a 6300.

****

Paul Christensen, W9AC

****,

Correct.  With reasonable care, you can have multiple transceivers that use open collectors and/ or open drains switch one common amp.  That arrangement works well as long as a rig's amp key line transistor does not use a pull-up resistor.  In those cases, a steering diode can be used to isolate multiple voltage sources.   

In the past, I have paralleled as many as six open collector outputs to switch one amplifier.  You can cascade as many RCA "Y" adapters as needed for the job.  To safely make this work, one needs to review and understand the transceiver's amp key line schematics.   

It makes sense to disclose this part of a transceiver's schematic as it means less trouble for us when we understand what we're doing, and fewer returns to the manufacturer from blown switching transistors.

Paul, W9AC