Connecting a Ludlum 177 to Radmon with the Data port?

I have just had a quick look at the manual for your counter ( here ) and it states that on the 9-pin D connector, pin 3 is ground and pin 5 gives a negative pulse output of ~-4V. It is very doable, but will depend on your skillset and how you want to go about it. Despite is being called a data port, it lacks an actual data stream. Besides pin 5 (pulse) the others are all just voltage rails/ancillary stuff, so pin 5 is all we have to work with.

There are a few options available for getting the counts online, three that I can think of right now are:

1. Use Radlog Pro on a windows PC and use the soundcard to pick up the pulses from your counter.
2. Connect the pulse output to a ESP32 or ESP8266 (Wemos D1 Mini etc).
3. Connect the pulse output to a Raspberry Pi.

All three options will require the ~-4V to be lowered to the max input voltage of the device you connect your counter to. For PC soundcard you want a max of +-1V. ESP32 should be 2.5-3.3V, same for the ESP8266, but the Wemos D1 Mini is a little more forgiving and should be fine with the ~-4V directly into it. I can't comment for RPi's as I have never interfaced a RPi with a counter. There is a ton of info out there though, and also on this forum (search: pyradmon). To lower the voltage a voltage divider would be the way to go. Two resistors, as simple as that. 

If going the ESP32/8266 route, I would strongly suggest a Wemos D1 mini and Steadramon's ESPGeiger firmware. I think this would be the easiest and fastest way of getting your counter online and it supports other services too. I reckon you could simply just hook up your pin 5 right to the Wemos, flash it with ESPGeiger, configure and go.