Got serial working.

cmd/gqgmc/main.go

@@ -57,13 +57,13 @@ func main() {
 		fmt.Printf("Failed to connect to geiger counter: '%s'\n", err)
 		return
 	}
-	fmt.Printf("Version: %s\n", ser)
+	fmt.Printf("Serial: %s\n", ser)
 
 	volts, err = gc.Volts()
 	if err != nil {
 		fmt.Printf("Failed to connect to geiger counter: '%s'\n", err)
 		return
 	}
-	fmt.Printf("Version: %d\n", volts)
+	fmt.Printf("Volts: %d\n", volts)
 
 }

devices/geiger/gqgmc.go

@@ -155,7 +155,7 @@ func (gc *GQGMCCounter) SerialNum() (string, error) {
 	serStr := ""
 
 	ser, err := gc.communicate(cmdGetSerial, 7)
-	if err != nil {
+	if err == nil {
 		for _, b := range ser {
 			serStr += fmt.Sprintf("%02X", b)
 		}