diff --git a/I2CManager_SAMD.h b/I2CManager_SAMD.h
new file mode 100644
index 0000000..aeabd6c
--- /dev/null
+++ b/I2CManager_SAMD.h
@@ -0,0 +1,188 @@
+/*
+ *  © 2021, Neil McKechnie. All rights reserved.
+ *
+ *  This file is part of CommandStation-EX
+ *
+ *  This is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  It is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with CommandStation.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef I2CMANAGER_SAMD_H
+#define I2CMANAGER_SAMD_H
+
+#include <Arduino.h>
+#include "I2CManager.h"
+
+//#include <avr/io.h>
+//#include <avr/interrupt.h>
+
+#if defined(I2C_USE_INTERRUPTS) && defined(ARDUINO_SAMD_ZERO)
+// PMA - IRQ handler, based on SERCOM3 being used for I2C, as per Arduino Zero & Sparkfun SAMD21
+// TODO: test
+void SERCOM3_Handler() {
+  I2CManagerClass::handleInterrupt();
+}
+#endif
+
+/***************************************************************************
+ *  Set I2C clock speed register.
+ ***************************************************************************/
+void I2CManagerClass::I2C_setClock(unsigned long i2cClockSpeed) {
+  unsigned long temp = ((F_CPU / i2cClockSpeed) - 16) / 2;
+  for (uint8_t preScaler = 0; preScaler<=3; preScaler++) {
+    if (temp <= 255) {
+      TWBR = temp;
+      TWSR = (TWSR & 0xfc) | preScaler;
+      return;
+    } else 
+      temp /= 4;
+  }
+  // Set slowest speed ~= 500 bits/sec
+  TWBR = 255;
+  TWSR |= 0x03;
+}
+
+/***************************************************************************
+ *  Initialise I2C registers.
+ ***************************************************************************/
+void I2CManagerClass::I2C_init()
+{
+  // PMA - broadly we do the following
+  initialise the clock
+  initialise the NVIC
+  software reset the I2C for the sercom
+  set master mode
+  do we need smart mode and quick command??
+  configure interrupt handlers
+  enable interrupts
+  set default baud rate
+  set SDA/SCL pins as outputs and enable pullups
+}
+
+/***************************************************************************
+ *  Initiate a start bit for transmission.
+ ***************************************************************************/
+void I2CManagerClass::I2C_sendStart() {
+  bytesToSend = currentRequest->writeLen;
+  bytesToReceive = currentRequest->readLen;
+  // We may have initiated a stop bit before this without waiting for it.
+  // Wait for stop bit to be sent before sending start.
+  while (TWCR & (1<<TWSTO)) {}
+  TWCR = (1<<TWEN)|ENABLE_TWI_INTERRUPT|(1<<TWINT)|(1<<TWEA)|(1<<TWSTA);  // Send Start
+}
+
+/***************************************************************************
+ *  Initiate a stop bit for transmission (does not interrupt)
+ ***************************************************************************/
+void I2CManagerClass::I2C_sendStop() {
+  TWDR = 0xff;  // Default condition = SDA released
+  TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA)|(1<<TWSTO);  // Send Stop
+}
+
+/***************************************************************************
+ *  Close I2C down
+ ***************************************************************************/
+void I2CManagerClass::I2C_close() {
+  // disable TWI
+  I2C_sendStop();
+  while (TWCR & (1<<TWSTO)) {}
+  TWCR = (1<<TWINT);                 // clear any interrupt and stop twi.
+}
+
+/***************************************************************************
+ *  Main state machine for I2C, called from interrupt handler or,
+ *  if I2C_USE_INTERRUPTS isn't defined, from the I2CManagerClass::loop() function
+ *  (and therefore, indirectly, from I2CRB::wait() and I2CRB::isBusy()).
+ ***************************************************************************/
+void I2CManagerClass::I2C_handleInterrupt() {
+  if (!(TWCR & (1<<TWINT))) return;  // Nothing to do.
+
+  uint8_t twsr = TWSR & 0xF8;
+
+  // Cases are ordered so that the most frequently used ones are tested first.
+  switch (twsr) {
+    case TWI_MTX_DATA_ACK:      // Data byte has been transmitted and ACK received
+    case TWI_MTX_ADR_ACK:       // SLA+W has been transmitted and ACK received
+      if (bytesToSend) {  // Send first.
+        if (operation == OPERATION_SEND_P)
+          TWDR = GETFLASH(currentRequest->writeBuffer + (txCount++));
+        else
+          TWDR = currentRequest->writeBuffer[txCount++];
+        bytesToSend--;
+        TWCR = (1<<TWEN)|ENABLE_TWI_INTERRUPT|(1<<TWINT)|(1<<TWEA);
+      } else if (bytesToReceive) {  // All sent, anything to receive?
+        while (TWCR & (1<<TWSTO)) {}    // Wait for stop to be sent
+        TWCR = (1<<TWEN)|ENABLE_TWI_INTERRUPT|(1<<TWINT)|(1<<TWEA)|(1<<TWSTA);  // Send Start
+      } else {  // Nothing left to send or receive
+        TWDR = 0xff;  // Default condition = SDA released
+        TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA)|(1<<TWSTO);  // Send Stop
+        state = I2C_STATUS_OK;
+      }
+      break;
+    case TWI_MRX_DATA_ACK:      // Data byte has been received and ACK transmitted
+      if (bytesToReceive > 0) {
+        currentRequest->readBuffer[rxCount++] = TWDR;
+        bytesToReceive--;
+      }
+      /* fallthrough */
+    case TWI_MRX_ADR_ACK:      // SLA+R has been sent and ACK received
+      if (bytesToReceive <= 1) {
+        TWCR = (1<<TWEN)|ENABLE_TWI_INTERRUPT|(1<<TWINT); // Send NACK after next reception
+      } else {
+        // send ack
+        TWCR = (1<<TWEN)|ENABLE_TWI_INTERRUPT|(1<<TWINT)|(1<<TWEA);
+      }
+      break;
+    case TWI_MRX_DATA_NACK:     // Data byte has been received and NACK transmitted
+      if (bytesToReceive > 0) {
+        currentRequest->readBuffer[rxCount++] = TWDR;
+        bytesToReceive--;
+      }
+      TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA)|(1<<TWSTO);  // Send Stop
+      state = I2C_STATUS_OK;
+      break;
+    case TWI_START:             // START has been transmitted  
+    case TWI_REP_START:         // Repeated START has been transmitted
+      // Set up address and R/W
+      if (operation == OPERATION_READ || (operation==OPERATION_REQUEST && !bytesToSend))
+        TWDR = (currentRequest->i2cAddress << 1) | 1; // SLA+R
+      else
+        TWDR = (currentRequest->i2cAddress << 1) | 0; // SLA+W
+      TWCR = (1<<TWEN)|ENABLE_TWI_INTERRUPT|(1<<TWINT)|(1<<TWEA);
+      break;
+    case TWI_MTX_ADR_NACK:      // SLA+W has been transmitted and NACK received
+    case TWI_MRX_ADR_NACK:      // SLA+R has been transmitted and NACK received
+    case TWI_MTX_DATA_NACK:     // Data byte has been transmitted and NACK received
+      TWDR = 0xff;  // Default condition = SDA released
+      TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA)|(1<<TWSTO);  // Send Stop
+      state = I2C_STATUS_NEGATIVE_ACKNOWLEDGE;
+      break;
+    case TWI_ARB_LOST:          // Arbitration lost
+      // Restart transaction from start.
+      I2C_sendStart();
+      break;
+    case TWI_BUS_ERROR:         // Bus error due to an illegal START or STOP condition
+    default:
+      TWDR = 0xff;  // Default condition = SDA released
+      TWCR = (1<<TWEN)|(1<<TWINT)|(1<<TWEA)|(1<<TWSTO);  // Send Stop
+      state = I2C_STATUS_TRANSMIT_ERROR;
+  }
+}
+
+#if defined(I2C_USE_INTERRUPTS)
+ISR(TWI_vect) {
+  I2CManagerClass::handleInterrupt();
+}
+#endif
+
+#endif /* I2CMANAGER_AVR_H */
diff --git a/defines.h b/defines.h
index f48811c..c24d6af 100644
--- a/defines.h
+++ b/defines.h
@@ -79,8 +79,6 @@
 //
 #define WIFI_SERIAL_LINK_SPEED 115200
 
-// TODO: PMA - figure out why enabling this causes the CS to crashe immediately after starting the motor driver
-//             on the SAMD platform - going to try to rebase to current TrackManager in case it's not my bug :-)
 #if __has_include ( "myAutomation.h")
   #if defined(BIG_RAM) || defined(DISABLE_EEPROM)
     #define EXRAIL_ACTIVE