diff --git a/klippy/klippy.py b/klippy/klippy.py
index 7af2e9b8..92fe6363 100644
--- a/klippy/klippy.py
+++ b/klippy/klippy.py
@@ -6,6 +6,7 @@
 # This file may be distributed under the terms of the GNU GPLv3 license.
 import sys, optparse, ConfigParser, logging, time, threading
 import gcode, toolhead, util, mcu, fan, heater, extruder, reactor, queuelogger
+import msgproto
 
 message_startup = """
 The klippy host software is attempting to connect.  Please
@@ -19,6 +20,15 @@ command to reload the config and restart the host software.
 Printer is halted
 """
 
+message_protocol_error = """
+This type of error is frequently caused by running an older
+version of the firmware on the micro-controller (fix by
+recompiling and flashing the firmware).
+Once the underlying issue is corrected, use the "RESTART"
+command to reload the config and restart the host software.
+Protocol error connecting to printer
+"""
+
 message_mcu_connect_error = """
 This is an unrecoverable error.  Please manually restart the
 micro-controller and then issue the "RESTART" command to
@@ -169,6 +179,10 @@ class Printer:
             logging.exception("Config error")
             self.state_message = "%s%s" % (str(e), message_restart)
             self.reactor.update_timer(self.stats_timer, self.reactor.NEVER)
+        except msgproto.error, e:
+            logging.exception("Protocol error")
+            self.state_message = "%s%s" % (str(e), message_protocol_error)
+            self.reactor.update_timer(self.stats_timer, self.reactor.NEVER)
         except mcu.error, e:
             logging.exception("MCU error during connect")
             self.state_message = "%s%s" % (str(e), message_mcu_connect_error)
diff --git a/klippy/mcu.py b/klippy/mcu.py
index 5d8fb31f..eddc56a5 100644
--- a/klippy/mcu.py
+++ b/klippy/mcu.py
@@ -283,7 +283,7 @@ class MCU_adc:
             minval = 0
         if maxval is None:
             maxval = 0xffff
-        mcu_adc_max = float(self._mcu.serial.msgparser.config["ADC_MAX"])
+        mcu_adc_max = self._mcu.serial.msgparser.get_constant_float("ADC_MAX")
         max_adc = sample_count * mcu_adc_max
         self._min_sample = int(minval * max_adc)
         self._max_sample = min(0xffff, int(math.ceil(maxval * max_adc)))
@@ -357,9 +357,9 @@ class MCU:
             self.serial.connect()
             self._printer.reactor.update_timer(
                 self._timeout_timer, time.time() + self.COMM_TIMEOUT)
-        self._mcu_freq = float(self.serial.msgparser.config['CLOCK_FREQ'])
-        self._stats_sumsq_base = float(
-            self.serial.msgparser.config['STATS_SUMSQ_BASE'])
+        self._mcu_freq = self.serial.msgparser.get_constant_float('CLOCK_FREQ')
+        self._stats_sumsq_base = self.serial.msgparser.get_constant_float(
+            'STATS_SUMSQ_BASE')
         self._emergency_stop_cmd = self.lookup_command("emergency_stop")
         self._clear_shutdown_cmd = self.lookup_command("clear_shutdown")
         self.register_msg(self.handle_shutdown, 'shutdown')
@@ -424,7 +424,7 @@ class MCU:
             self._num_oids,))
 
         # Resolve pin names
-        mcu = self.serial.msgparser.config['MCU']
+        mcu = self.serial.msgparser.get_constant('MCU')
         pin_map = self._config.get('pin_map', None)
         if pin_map is None:
             pnames = pins.mcu_to_pins(mcu)
diff --git a/klippy/msgproto.py b/klippy/msgproto.py
index 76dc0544..c9d14138 100644
--- a/klippy/msgproto.py
+++ b/klippy/msgproto.py
@@ -308,3 +308,15 @@ class MessageParser:
         self.static_strings = data.get('static_strings', [])
         self.config.update(data.get('config', {}))
         self.version = data.get('version', '')
+    def get_constant(self, name):
+        try:
+            return self.config[name]
+        except KeyError:
+            raise error("Firmware constant '%s' not found" % (name,))
+    def get_constant_float(self, name):
+        try:
+            return float(self.config[name])
+        except ValueError:
+            raise error("Firmware constant '%s' not a float" % (name,))
+        except KeyError:
+            raise error("Firmware constant '%s' not found" % (name,))