But, but, but... it helps to know the words to google! I googled RTCM GM300 and didn’t get the links you did. But I’m here to learn!

Not sure of this line.
 
GM300 Pin 11 - RTCM Pin 3 - Rx Audio JU 551 inside the GM300,  in position A gives FLAT (not De-emphasised) audio out pin 11.We want to use the RTCM's internal Squelch.
 
So pin 11 on the GM300 already has the flat audio we seek. But Jumper 551 to the A position is the best audio? OK. Buy why the RTCM’s squelch? I know that we’ve had problems with squelch tails...
 
From: Sam Nabkey
Sent: Saturday, June 20, 2015 5:06 AM
Subject: Re: [App_rpt-users] Wiring a GM300 to a RTCM
 
Hello..
 
GM300 Pin 11 - RTCM Pin 3 - Rx Audio JU 551 inside the GM300,  in position A gives FLAT (not De-emphasised) audio out pin 11.We want to use the RTCM's internal Squelch.
 
GM300 Pin 8 - RTCM  Pin 4 - CTCSS Input
GM300 Pin 5 - RTCM Pin 2 - TX Audio
GM300 Pin 3 - RTCM Pin 7 - PTT
GM300 Pin 7 - RTCM Pin 9 - Gnd
 
RTCM Info - http://www.micro-node.com/thin-m1.html
GM300 Info- http://www.batlabs.com/gm300.html
 
Google is your friend. 
 
Have fun!!! 
 
 
 
On Sat, Jun 20, 2015 at 1:48 AM, R. Wayne <allstar@controlservers.net> wrote:
I’ll confess that I am new to this part. It was dumped on me without warning and that’s OK. Time to learn a few things. I have a GM300 that is going to be a remote base and a cable purchased from eBay that is prewired for the GM300 side. I understand (I think) that TX goes RX, RX goes to TX, GRND to GRND – but now for PTT, COR and CTCSS. I wonder if some could push me over a cliff – I mean in the right direction. I have no documentation on the RTCM.
 
Thanks, R. Wayne
 
The RTCM connections are:
 
DB9-Female

1 Vin (12v)
2 TX Audio
3 RX Audio
4 CTCSS
5 GND
6 GND
7 PTT
8 GND
9 GND
 
The GM300 cable (and radio) connections are:
 
Pin #
 
2     MIC                                          BLU
3     PTT                                           WHI-BLU
4     PROGRAMMABLE OUTPUT     BRO
5     FLAT TX AUDIO                        GRN
7     GROUND                                  WHI-GRN 
8     PROGRAMMABLE I/O (COR ?)ONG
11   RECEIVE AUDIO                       WHI-ONG
14   PROGRAMMABLE I/O              WHI-BRO
15-16  Jumpered to enable internal speaker

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