modem signal check

README.md

Build:

GOOS=linux GOARCH=mipsle CGO_ENABLED=0 GOMIPS=softfloat go build

modem.go

package main

import (
	"fmt"
	"io"
	"log"
	"strconv"
	"strings"

	"github.com/jacobsa/go-serial/serial"
)

const (
	respPrefix = "^HCSQ: "
	numParts   = 5
)

// ^HCSQ: "LTE",41,36,141,28

func processResponse(s string) {
	if !strings.HasPrefix(s, respPrefix) {
		return
	}
	parts := strings.Split(s[len(respPrefix):], ",")
	if parts[0] != `"LTE"` || len(parts) != numParts {
		fmt.Printf("RAW RESPONSE: %s\n", s)
		return
	}
	nums := make([]float64, len(parts)-1)
	for n := range nums {
		v, err := strconv.Atoi(parts[n+1])
		if err != nil {
			fmt.Printf("RAW RESPONSE: %s\n", s)
			return
		}
		nums[n] = float64(v)
	}

	// FIXME: handle 255="unknown or undetectable"
	// FIXME: these are actually ranges
	rssi := nums[0] - 120.0
	rsrp := nums[1] - 140.0
	sinr := nums[2]*0.2 - 20.0 + 0.1
	rsrq := nums[3]*0.5 - 19.5
	fmt.Printf("LTE: RSSI=%f RSRP=%f SINR=%f RSRQ=%f\n", rssi, rsrp, sinr, rsrq)
}

func main() {
	options := serial.OpenOptions{
		PortName:              "/dev/ttyUSB2",
		BaudRate:              115200,
		DataBits:              8,
		StopBits:              1,
		MinimumReadSize:       0,
		InterCharacterTimeout: 500,
	}

	port, err := serial.Open(options)
	if err != nil {
		log.Fatalf("serial.Open(): %v", err)
	}

	defer port.Close()
	for {
		if _, err = port.Write([]byte("AT^HCSQ?\r\n")); err != nil {
			log.Fatalf("port.Write(): %v", err)
		}

		b := make([]byte, 1)
		s := ""
	INNERLOOP:
		for {
			n, err := port.Read(b)
			switch {
			case err == io.EOF:
				processResponse(s)
				break INNERLOOP
			case err != nil:
				processResponse(s)
				log.Fatalf("port.Read(): %v", err)
			case n == 0:
				processResponse(s)
				log.Fatalf("port.Read(): no data")
			case b[0] == 13:
				// NOOP
			case b[0] == 10:
				processResponse(s)
				s = ""
			default:
				s += string(b)
			}
		}
	}
}

go-serial needs a patch (better use another fork, didn't bother to find it):

diff --git a/serial/open_linux.go b/serial/open_linux.go
index ccce12c..a27fa73 100644
--- a/serial/open_linux.go
+++ b/serial/open_linux.go
@@ -25,9 +25,10 @@ import (
 // }
 //
 const (
-	kTCSETS2 = 0x402C542B
-	kBOTHER  = 0x1000
-	kNCCS    = 19
+	// https://github.com/jacobsa/go-serial/issues/37#issuecomment-379078372
+	kTCSETS2 = 0x8030542B
+	kBOTHER  = 0x00001000
+	kNCCS    = 23
 )
 
 //