I've been working on my Raspberry Pi recently, trying to connect it to an Adafruit FONA cellular module. The primary communication has been through TTL serial between the two.
Recently, I reluctantly switched to USB connection, for speed, which freed up the serial pins on the Pi and on the FONA. I came across this article, "Add a 9-pin serial port to your Raspberry Pi in 10 Minutes" http://www.davidhunt.ie/add-a-9-pin-serial-port-to-your-raspberry-pi-in-10-minutes/ And It occurred to me that I could use one to connect my Pi, or a ZeroW, even, to my M100. Now I'm wondering if I can use it to connect the FONA directly to the M100, forgetting the Pi. It should be doable, since the needed pins are on the FONA. I think it would be cool to be able to log onto my Pi from my M100 (and I can think of several uses people might have for this setup), but even cooler to use the FONA. The FONA also has... uhm... I2C, I think it is, connectors. So other sensors could be connected to it via those connectors, and the M100 could be used as a remote data collection device. I think. Anyway, I picked up a couple RS232-TTL converters, with DB9s and using a Max3232 chip off of ebay for about $12 for both. You can get them a LOT cheaper (under $3.00 US for one), but these gave me the confidence that they wouldn't fail on me. I'll post here any experiences I have once I play with it some. Just thought I'd share on something that was new to me.