diff --git a/RFout1MHzV1_05/RFout1MHzV1_05.ino b/RFout1MHzV1_05/RFout1MHzV1_05.ino
index 985a752..cd039ab 100644
--- a/RFout1MHzV1_05/RFout1MHzV1_05.ino
+++ b/RFout1MHzV1_05/RFout1MHzV1_05.ino
@@ -14,12 +14,25 @@ SoftwareSerial gpsPort(2, 3); // RX, TX
 #define LEDIndicator1 5  //LED indicator for GPS Lock on pin A3
 #define FIXOut 4         //Pin Out at IDC. Indicator for GPS Lock on pin 4
 #define LDO_Enable A3    //GPS Voltage regulator Enable on pin A3
-boolean GPSOK;
-const char softwareversion[] = "1.05" ; //Version of this program, sent to serialport at startup
+boolean GPSOK, passthrough;
+int state; // serial command state machine
+uint32_t freq; // Frequency in Hz commanded via serial
+#define softwareversion "1.05" //Version of this program, sent to serialport at startup
 NMEAGPS  gps; // This parses the GPS characters
 gps_fix  fix; // This holds on to the latest values
 
+constexpr auto MHz = 1000000lu;
+constexpr auto KHz = 1000lu;
+constexpr auto Hz = 1lu;
 
+// This is a little bit arcane but it just lets you keep the user string
+// and the actual value in synch automatically
+#define SPEED 1
+#define UNITS MHz
+#define STRINGIFY1(x) #x
+#define STRINGIFY(x) STRINGIFY1(x)
+#define SPEED_STR STRINGIFY(SPEED) STRINGIFY(UNITS)
+#define SPEED_ARG (UINT32_C(SPEED) * UNITS)
 
 //--------------------------  SETUP -----------------------
 
@@ -27,9 +40,18 @@ void setup()
 {
   Serial.begin(9600);
   while (!Serial);
-  Serial.println("");
-  Serial.print(F("Zachtek GPS referenced RF, Software version: "));
-  Serial.println(softwareversion);
+
+  Serial.println(F(""));
+  Serial.println(F("Zachtek GPS referenced RF, Software version: " softwareversion));
+
+  if(strapped_for_passthrough()) {
+    pinMode(LDO_Enable, OUTPUT); // Set Voltage Regulator Enable pin as output.
+    digitalWrite(LDO_Enable, HIGH); //Turn on 3.1V Power supply for the Ublox GPS module
+    gpsPort.begin(9600);
+    passthrough = 1;
+    Serial.println(F("Strapped for passthrough.  Disconnect jumper between SDA/SCL for normal operation"));
+    return;
+  }
 
   pinMode(LDO_Enable, OUTPUT); // Set Voltage Regulator Enable pin as output.
 
@@ -58,15 +80,15 @@ void setup()
   delay(500);//Wait for GPSmodule to complete it's power on.
 
   //Program GPS to output RF
-  if (setGPS_OutputFreq1MHz()) {
-    Serial.println ("GPS Initialized to output RF at 1MHz");
+  if (setGPS_OutputFreq(SPEED_ARG)) {
+    Serial.println ("GPS Initialized to output RF at " SPEED_STR);
     Serial.println ("Initialization is complete.");
     Serial.println ("");
     GPSOK = true;
   }
   else
   {
-    Serial.println ("Error! Could not program GPS!");
+    Serial.println (F("Error! Could not program GPS!"));
     GPSOK = false;
   }
 }
@@ -74,11 +96,59 @@ void setup()
 //--------------------------
 
 
+// Handle serial commands
+void handle_serial() {
+  int c = Serial.read();
+  if(c == 'P') {
+    Serial.println(F("Entering passthrough mode -- reset microcontroller to return to normal mode\n"));
+    passthrough = true;
+  }
+  else if(c == 'F' || c == 'f') {
+    Serial.print(F("Frequency?"));
+    state = 1; freq = 0;
+  }
+  else if(state == 1) {
+    if(c >= '0' && c <= '9') {
+      Serial.write(c);
+      freq = freq * 10 + c - '0';
+    } else if (c == 'M' || c == 'm') {
+      Serial.write(c);
+      freq *= 1000000lu;
+    } else if (c == 'K' || c == 'k') {
+      Serial.write(c);
+      freq *= 1000lu;
+    } else if (c == '\n' || c == '\r') {
+      Serial.println(F(""));
+      if (setGPS_OutputFreq(freq)) {
+        Serial.print ("GPS Initialized to output RF at " );
+        Serial.println (freq);
+        Serial.println ("Initialization is complete.");
+        Serial.println ("");
+        GPSOK = true;
+      }
+      else
+      {
+        Serial.println (F("Error! Could not program GPS!"));
+        GPSOK = false;
+      }
+      state = 0;
+    }
+  }
+}
 
 
 //--------------------------  Main loop -----------------------
 void loop()
 {
+  if(passthrough) {
+    if(gpsPort.available()) Serial.write(gpsPort.read());
+    if(Serial.available()) gpsPort.write(Serial.read());
+    return;
+  }
+  if(Serial.available()) {
+    handle_serial();
+  }
+  if(state != 0) return;
   while (gps.available( gpsPort )) {
     fix = gps.read();
     if (fix.valid.location && fix.valid.date && fix.valid.time)
@@ -108,81 +178,68 @@ void loop()
 
 //--------------------------
 
-bool setGPS_OutputFreq100kHz()
-{
-  int gps_set_sucess = 0;
-  uint8_t setOutputFreq[] = {
-    0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x32, 0x00, 0x00, 0x00, 0x01, 0x00,
-    0x00, 0x00, 0xA0, 0x86, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
-    0x00, 0x00, 0xEF, 0x00, 0x00, 0x00, 0x20, 0x1B
-  };
+// For details of the UBX protocol, see
+// https://www.u-blox.com/sites/default/files/products/documents/u-blox7-V14_ReceiverDescriptionProtocolSpec_%28GPS.G7-SW-12001%29_Public.pdf
+// Documentation of this packet is under the heading CFG-TP5 (35.19.2) in the current documentation.
+uint8_t setOutputFreq[] = {
+0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x32, 0x00, 0x00, 0x00, 0x01, 0x00,
+0x00, 0x00, 0xA0, 0x86, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
+0x00, 0x00, 0xEF, 0x00, 0x00, 0x00, 0x20, 0x1B
+};
 
+#define OFFSET_FREQUENCY_LOCKED (18)
+#define OFFSET_CKSUM (38)
+
+// Note: Normally payload_start is 2 bytes past the start of buf, because the first two bytes
+// are the message type and are not checksummed.
+void ubx_compute_checksum(uint8_t *payload_start, uint8_t *payload_end, uint8_t *cksum) {
+  uint8_t ck_a=0, ck_b=0;
+  for(const uint8_t *p = payload_start; p != payload_end; p++)
+  {
+    ck_a += *p;
+    ck_b += ck_a;
+  }
+  cksum[0] = ck_a;
+  cksum[1] = ck_b;
+}
+
+bool setGPS_OutputFreq(uint32_t freq) {
+  for(int i=0; i<4; i++) {
+    setOutputFreq[OFFSET_FREQUENCY_LOCKED+i] = freq & 0xff;
+    freq >>= 8;
+  }
+  ubx_compute_checksum(setOutputFreq+2, setOutputFreq+38, setOutputFreq+38);
   sendUBX(setOutputFreq, sizeof(setOutputFreq) / sizeof(uint8_t));
-  gps_set_sucess = getUBX_ACK(setOutputFreq);
-  //Serial.println("Set output Freq Done");
+  bool gps_set_sucess = getUBX_ACK(setOutputFreq);
+  // Serial.println(F("Set output Freq Done"));
   return gps_set_sucess;
 }
 
+bool setGPS_OutputFreq1Kz()
+{
+  return setGPS_OutputFreq(1*KHz);
+}
+
 bool setGPS_OutputFreq1MHz()
 {
-  int gps_set_sucess = 0;
-  uint8_t setOutputFreq[] = {
-    0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x32, 0x00, 0x00, 0x00, 0x01, 0x00,
-    0x00, 0x00, 0x40, 0x42, 0x0F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
-    0x00, 0x00, 0xEF, 0x00, 0x00, 0x00, 0x8A, 0x8B
-  };
-
-  sendUBX(setOutputFreq, sizeof(setOutputFreq) / sizeof(uint8_t));
-  gps_set_sucess = getUBX_ACK(setOutputFreq);
-  //Serial.println("Set output Freq Done");
-  return gps_set_sucess;
+  return setGPS_OutputFreq(1*MHz);
 }
 
 bool setGPS_OutputFreq2MHz()
 {
-  int gps_set_sucess = 0;
-  uint8_t setOutputFreq[] = {
-    0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x32, 0x00, 0x00, 0x00, 0x01, 0x00,
-    0x00, 0x00, 0x80, 0x84, 0x1E, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
-    0x00, 0x00, 0xEF, 0x00, 0x00, 0x00, 0x1B, 0x7F
-  };
-
-  sendUBX(setOutputFreq, sizeof(setOutputFreq) / sizeof(uint8_t));
-  gps_set_sucess = getUBX_ACK(setOutputFreq);
-  //Serial.println("Set output Freq Done");
-  return gps_set_sucess;
+  return setGPS_OutputFreq(2*MHz);
 }
 
 
 bool setGPS_OutputFreq4MHz()
 {
-  int gps_set_sucess = 0;
-  uint8_t setOutputFreq[] = {
-    0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x32, 0x00, 0x00, 0x00, 0x01, 0x00,
-    0x00, 0x00, 0x00, 0x09, 0x3D, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
-    0x00, 0x00, 0xEF, 0x00, 0x00, 0x00, 0x3F, 0x8C
-  };
-
-  sendUBX(setOutputFreq, sizeof(setOutputFreq) / sizeof(uint8_t));
-  gps_set_sucess = getUBX_ACK(setOutputFreq);
-  //Serial.println("Set output Freq Done");
-  return gps_set_sucess;
+  return setGPS_OutputFreq(4*MHz);
 }
 
 //8MHz is the highest low-jitter frequency possible
 bool setGPS_OutputFreq8MHz()
 {
-  int gps_set_sucess = 0;
-  uint8_t setOutputFreq[] = {
-    0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
-    0x00, 0x00, 0x00, 0x12, 0x7A, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
-    0x00, 0x00, 0x6F, 0x00, 0x00, 0x00, 0xD4, 0x28
-  };
-
-  sendUBX(setOutputFreq, sizeof(setOutputFreq) / sizeof(uint8_t));
-  gps_set_sucess = getUBX_ACK(setOutputFreq);
-  //Serial.println("Set output Freq Done");
-  return gps_set_sucess;
+  return setGPS_OutputFreq(8*MHz);
 }
 
 //10 MHz is very jittery. Numbers that can be done with an integer division from 48MHz will produce
@@ -190,34 +247,14 @@ bool setGPS_OutputFreq8MHz()
 //If 10MHz low jitter is needed then one option is to output 2MHz and then filter out the 5th overtone arriving at 10MHz in that way.
 bool setGPS_OutputFreq10MHz()
 {
-  int gps_set_sucess = 0;
-  uint8_t setOutputFreq[] = {
-    0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
-    0x00, 0x00, 0x80, 0x96, 0x98, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
-    0x00, 0x00, 0x6F, 0x00, 0x00, 0x00, 0xF6, 0x10
-  };
-
-  sendUBX(setOutputFreq, sizeof(setOutputFreq) / sizeof(uint8_t));
-  gps_set_sucess = getUBX_ACK(setOutputFreq);
-  //Serial.println("Set output Freq Done");
-  return gps_set_sucess;
+  return setGPS_OutputFreq(10*MHz);
 }
 
 //16MHz is above the specs for lUblox Neo-6, only included for experiments.
 //This will not produce as clean Square wave.
 bool setGPS_OutputFreq16MHz()
 {
-  int gps_set_sucess = 0;
-  uint8_t setOutputFreq[] = {
-    0xB5, 0x62, 0x06, 0x31, 0x20, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
-    0x00, 0x00, 0x00, 0x24, 0xF4, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00,
-    0x00, 0x00, 0x6F, 0x00, 0x00, 0x00, 0x60, 0x12
-  };
-
-  sendUBX(setOutputFreq, sizeof(setOutputFreq) / sizeof(uint8_t));
-  gps_set_sucess = getUBX_ACK(setOutputFreq);
-  //Serial.println("Set output Freq Done");
-  return gps_set_sucess;
+  return setGPS_OutputFreq(16*MHz);
 }
 
 
@@ -249,10 +286,7 @@ boolean getUBX_ACK(uint8_t *MSG) {
   ackPacket[9] = 0; // CK_B
 
   // Calculate the checksums
-  for (uint8_t ubxi = 2; ubxi < 8; ubxi++) {
-    ackPacket[8] = ackPacket[8] + ackPacket[ubxi];
-    ackPacket[9] = ackPacket[9] + ackPacket[8];
-  }
+  ubx_compute_checksum(ackPacket+2, ackPacket+8, ackPacket+8);
 
   while (1) {  // Test for success
     if (ackByteID > 9) {
@@ -277,4 +311,26 @@ boolean getUBX_ACK(uint8_t *MSG) {
       }//else
     }//If
   }//While
-}//getUBX_ACK
+}//getUBX_ACK
+
+// If pins 7/8 on the 10-pin header are bridged, this is "strapped for passthrough"
+// Detect this by driving one pin high and checking the other pin, then driving low and repeating
+// set pins back to inputs before returning
+bool strapped_for_passthrough() {
+  bool result = true;
+  pinMode(A4, OUTPUT);
+
+  digitalWrite(A4, HIGH);
+  delay(1);
+  if(!digitalRead(A5)) result = false;
+
+  digitalWrite(A4, LOW);
+  pinMode(A5, INPUT_PULLUP);
+  delay(1);
+  if(digitalRead(A5)) result = false;
+
+  pinMode(A4, INPUT);
+  pinMode(A5, INPUT);
+
+  return result;
+}