diff --git a/contracts/interfaces/ISTO.sol b/contracts/interfaces/ISTO.sol
new file mode 100644
index 000000000..dd5ce3f16
--- /dev/null
+++ b/contracts/interfaces/ISTO.sol
@@ -0,0 +1,11 @@
+pragma solidity ^0.4.24;
+
+/**
+ * @title Interface to be implemented by all STO modules
+ */
+interface ISTO {
+    /**
+     * @notice Returns the total no. of tokens sold
+     */
+    function getTokensSold() external view returns (uint256);
+}
diff --git a/contracts/modules/STO/CappedSTO.sol b/contracts/modules/STO/CappedSTO.sol
index af091dacf..3245aa3f5 100644
--- a/contracts/modules/STO/CappedSTO.sol
+++ b/contracts/modules/STO/CappedSTO.sol
@@ -1,6 +1,6 @@
 pragma solidity ^0.4.24;
 
-import "./ISTO.sol";
+import "./STO.sol";
 import "../../interfaces/ISecurityToken.sol";
 import "openzeppelin-solidity/contracts/ReentrancyGuard.sol";
 import "openzeppelin-solidity/contracts/math/SafeMath.sol";
@@ -8,7 +8,7 @@ import "openzeppelin-solidity/contracts/math/SafeMath.sol";
 /**
  * @title STO module for standard capped crowdsale
  */
-contract CappedSTO is ISTO, ReentrancyGuard {
+contract CappedSTO is STO, ReentrancyGuard {
     using SafeMath for uint256;
 
     // Determine whether users can invest on behalf of a beneficiary
@@ -51,7 +51,7 @@ contract CappedSTO is ISTO, ReentrancyGuard {
      * @param _startTime Unix timestamp at which offering get started
      * @param _endTime Unix timestamp at which offering get ended
      * @param _cap Maximum No. of token base units for sale
-     * @param _rate Token units a buyer gets multiplied by 10^18 per wei / base unit of POLY 
+     * @param _rate Token units a buyer gets multiplied by 10^18 per wei / base unit of POLY
      * @param _fundRaiseTypes Type of currency used to collect the funds
      * @param _fundsReceiver Ethereum account address to hold the funds
      */
@@ -115,7 +115,6 @@ contract CappedSTO is ISTO, ReentrancyGuard {
         weiAmount = weiAmount.sub(refund);
 
         _forwardFunds(refund);
-        _postValidatePurchase(_beneficiary, weiAmount);
     }
 
     /**
@@ -127,7 +126,6 @@ contract CappedSTO is ISTO, ReentrancyGuard {
         require(fundRaiseTypes[uint8(FundRaiseType.POLY)], "Mode of investment is not POLY");
         uint256 refund = _processTx(msg.sender, _investedPOLY);
         _forwardPoly(msg.sender, wallet, _investedPOLY.sub(refund));
-        _postValidatePurchase(msg.sender, _investedPOLY.sub(refund));
     }
 
     /**
@@ -161,7 +159,7 @@ contract CappedSTO is ISTO, ReentrancyGuard {
      * @return Token units a buyer gets(multiplied by 10^18) per wei / base unit of POLY
      * @return Amount of funds raised
      * @return Number of individual investors this STO have.
-     * @return Amount of tokens get sold. 
+     * @return Amount of tokens get sold.
      * @return Boolean value to justify whether the fund raise type is POLY or not, i.e true for POLY.
      */
     function getSTODetails() public view returns(uint256, uint256, uint256, uint256, uint256, uint256, uint256, bool) {
@@ -203,8 +201,6 @@ contract CappedSTO is ISTO, ReentrancyGuard {
 
         _processPurchase(_beneficiary, tokens);
         emit TokenPurchase(msg.sender, _beneficiary, _investedAmount, tokens);
-
-        _updatePurchasingState(_beneficiary, _investedAmount);
     }
 
     /**
@@ -216,21 +212,10 @@ contract CappedSTO is ISTO, ReentrancyGuard {
     function _preValidatePurchase(address _beneficiary, uint256 _investedAmount) internal view {
         require(_beneficiary != address(0), "Beneficiary address should not be 0x");
         require(_investedAmount != 0, "Amount invested should not be equal to 0");
-        uint256 tokens;
-        (tokens, ) = _getTokenAmount(_investedAmount);
-        require(totalTokensSold.add(tokens) <= cap, "Investment more than cap is not allowed");
         /*solium-disable-next-line security/no-block-members*/
         require(now >= startTime && now <= endTime, "Offering is closed/Not yet started");
     }
 
-    /**
-    * @notice Validation of an executed purchase.
-      Observe state and use revert statements to undo rollback when valid conditions are not met.
-    */
-    function _postValidatePurchase(address /*_beneficiary*/, uint256 /*_investedAmount*/) internal pure {
-      // optional override
-    }
-
     /**
     * @notice Source of tokens.
       Override this method to modify the way in which the crowdsale ultimately gets and sends its tokens.
@@ -255,27 +240,23 @@ contract CappedSTO is ISTO, ReentrancyGuard {
         _deliverTokens(_beneficiary, _tokenAmount);
     }
 
-    /**
-    * @notice Overrides for extensions that require an internal state to check for validity
-      (current user contributions, etc.)
-    */
-    function _updatePurchasingState(address /*_beneficiary*/, uint256 /*_investedAmount*/) internal pure {
-      // optional override
-    }
-
     /**
     * @notice Overrides to extend the way in which ether is converted to tokens.
     * @param _investedAmount Value in wei to be converted into tokens
     * @return Number of tokens that can be purchased with the specified _investedAmount
     * @return Remaining amount that should be refunded to the investor
     */
-    function _getTokenAmount(uint256 _investedAmount) internal view returns (uint256 _tokens, uint256 _refund) {
-        _tokens = _investedAmount.mul(rate);
-        _tokens = _tokens.div(uint256(10) ** 18);
+    function _getTokenAmount(uint256 _investedAmount) internal view returns (uint256 tokens, uint256 refund) {
+        tokens = _investedAmount.mul(rate);
+        tokens = tokens.div(uint256(10) ** 18);
+        if (totalTokensSold.add(tokens) > cap) {
+            tokens = cap.sub(totalTokensSold);
+        }
         uint256 granularity = ISecurityToken(securityToken).granularity();
-        _tokens = _tokens.div(granularity);
-        _tokens = _tokens.mul(granularity);
-        _refund = _investedAmount.sub((_tokens.mul(uint256(10) ** 18)).div(rate));
+        tokens = tokens.div(granularity);
+        tokens = tokens.mul(granularity);
+        require(tokens > 0, "Cap reached");
+        refund = _investedAmount.sub((tokens.mul(uint256(10) ** 18)).div(rate));
     }
 
     /**
diff --git a/contracts/modules/STO/DummySTO.sol b/contracts/modules/STO/DummySTO.sol
index 1b44d2e9b..fafc6c1c4 100644
--- a/contracts/modules/STO/DummySTO.sol
+++ b/contracts/modules/STO/DummySTO.sol
@@ -1,12 +1,12 @@
 pragma solidity ^0.4.24;
 
-import "./ISTO.sol";
+import "./STO.sol";
 import "../../interfaces/ISecurityToken.sol";
 
 /**
  * @title STO module for sample implementation of a different crowdsale module
  */
-contract DummySTO is ISTO {
+contract DummySTO is STO {
 
     bytes32 public constant ADMIN = "ADMIN";
 
diff --git a/contracts/modules/STO/PreSaleSTO.sol b/contracts/modules/STO/PreSaleSTO.sol
index 7378fb06b..4dacaa2d7 100644
--- a/contracts/modules/STO/PreSaleSTO.sol
+++ b/contracts/modules/STO/PreSaleSTO.sol
@@ -1,13 +1,13 @@
 pragma solidity ^0.4.24;
 
-import "./ISTO.sol";
+import "./STO.sol";
 import "../../interfaces/ISecurityToken.sol";
 import "openzeppelin-solidity/contracts/math/SafeMath.sol";
 
 /**
  * @title STO module for private presales
  */
-contract PreSaleSTO is ISTO {
+contract PreSaleSTO is STO {
     using SafeMath for uint256;
 
     bytes32 public constant PRE_SALE_ADMIN = "PRE_SALE_ADMIN";
diff --git a/contracts/modules/STO/ISTO.sol b/contracts/modules/STO/STO.sol
similarity index 87%
rename from contracts/modules/STO/ISTO.sol
rename to contracts/modules/STO/STO.sol
index 6fb216566..5f9a6e9e3 100644
--- a/contracts/modules/STO/ISTO.sol
+++ b/contracts/modules/STO/STO.sol
@@ -3,17 +3,18 @@ pragma solidity ^0.4.24;
 import "../../Pausable.sol";
 import "../Module.sol";
 import "../../interfaces/IERC20.sol";
-import "./ISTOStorage.sol";
+import "../../interfaces/ISTO.sol";
+import "./STOStorage.sol";
 import "openzeppelin-solidity/contracts/math/SafeMath.sol";
 
 /**
  * @title Interface to be implemented by all STO modules
  */
-contract ISTO is ISTOStorage, Module, Pausable  {
+contract STO is ISTO, STOStorage, Module, Pausable  {
     using SafeMath for uint256;
 
     enum FundRaiseType { ETH, POLY, SC }
-    
+
     // Event
     event SetFundRaiseTypes(FundRaiseType[] _fundRaiseTypes);
 
@@ -36,11 +37,6 @@ contract ISTO is ISTOStorage, Module, Pausable  {
         return fundsRaised[uint8(_fundRaiseType)];
     }
 
-    /**
-     * @notice Returns the total no. of tokens sold
-     */
-    function getTokensSold() public view returns (uint256);
-
     /**
      * @notice Pause (overridden function)
      */
@@ -59,7 +55,7 @@ contract ISTO is ISTOStorage, Module, Pausable  {
 
     function _setFundRaiseType(FundRaiseType[] _fundRaiseTypes) internal {
         // FundRaiseType[] parameter type ensures only valid values for _fundRaiseTypes
-        require(_fundRaiseTypes.length > 0, "Raise type is not specified");
+        require(_fundRaiseTypes.length > 0 && _fundRaiseTypes.length <= 3, "Raise type is not specified");
         fundRaiseTypes[uint8(FundRaiseType.ETH)] = false;
         fundRaiseTypes[uint8(FundRaiseType.POLY)] = false;
         fundRaiseTypes[uint8(FundRaiseType.SC)] = false;
diff --git a/contracts/modules/STO/ISTOStorage.sol b/contracts/modules/STO/STOStorage.sol
similarity index 88%
rename from contracts/modules/STO/ISTOStorage.sol
rename to contracts/modules/STO/STOStorage.sol
index b808c98b4..2e2401fb0 100644
--- a/contracts/modules/STO/ISTOStorage.sol
+++ b/contracts/modules/STO/STOStorage.sol
@@ -1,9 +1,9 @@
 pragma solidity ^0.4.24;
 
 /**
- * @title Storage layout for the ISTO contract
+ * @title Storage layout for the STO contract
  */
-contract ISTOStorage {
+contract STOStorage {
 
     mapping (uint8 => bool) public fundRaiseTypes;
     mapping (uint8 => uint256) public fundsRaised;
@@ -21,4 +21,4 @@ contract ISTOStorage {
      // Final amount of tokens sold
     uint256 public totalTokensSold;
 
-}
\ No newline at end of file
+}
diff --git a/contracts/modules/STO/USDTieredSTO.sol b/contracts/modules/STO/USDTieredSTO.sol
index f1c959671..c29402355 100644
--- a/contracts/modules/STO/USDTieredSTO.sol
+++ b/contracts/modules/STO/USDTieredSTO.sol
@@ -1,6 +1,6 @@
 pragma solidity ^0.4.24;
 
-import "./ISTO.sol";
+import "./STO.sol";
 import "../../interfaces/ISecurityToken.sol";
 import "../../interfaces/IOracle.sol";
 import "../../RegistryUpdater.sol";
@@ -12,7 +12,7 @@ import "../../storage/USDTieredSTOStorage.sol";
 /**
  * @title STO module for standard capped crowdsale
  */
-contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
+contract USDTieredSTO is USDTieredSTOStorage, STO, ReentrancyGuard {
     using SafeMath for uint256;
 
     string public constant POLY_ORACLE = "PolyUsdOracle";
@@ -262,7 +262,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
         }
         usdTokens = _usdTokens;
         for(i = 0; i < _usdTokens.length; i++) {
-            require(_usdTokens[i] != address(0), "Invalid USD token");
+            require(_usdTokens[i] != address(0) && _usdTokens[i] != address(polyToken), "Invalid USD token");
             usdTokenEnabled[_usdTokens[i]] = true;
         }
         emit SetAddresses(wallet, reserveWallet, _usdTokens);
@@ -276,7 +276,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @notice Finalizes the STO and mint remaining tokens to reserve address
      * @notice Reserve address must be whitelisted to successfully finalize
      */
-    function finalize() public onlyOwner {
+    function finalize() external onlyOwner {
         require(!isFinalized, "STO is already finalized");
         isFinalized = true;
         uint256 tempReturned;
@@ -301,7 +301,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @param _investors Array of investor addresses to modify
      * @param _accredited Array of bools specifying accreditation status
      */
-    function changeAccredited(address[] _investors, bool[] _accredited) public onlyOwner {
+    function changeAccredited(address[] _investors, bool[] _accredited) external onlyOwner {
         require(_investors.length == _accredited.length, "Array length mismatch");
         for (uint256 i = 0; i < _investors.length; i++) {
             if (_accredited[i]) {
@@ -319,7 +319,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @param _investors Array of investor addresses to modify
      * @param _nonAccreditedLimit Array of uints specifying non-accredited limits
      */
-    function changeNonAccreditedLimit(address[] _investors, uint256[] _nonAccreditedLimit) public onlyOwner {
+    function changeNonAccreditedLimit(address[] _investors, uint256[] _nonAccreditedLimit) external onlyOwner {
         //nonAccreditedLimitUSDOverride
         require(_investors.length == _nonAccreditedLimit.length, "Array length mismatch");
         for (uint256 i = 0; i < _investors.length; i++) {
@@ -357,7 +357,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @notice Function to set allowBeneficialInvestments (allow beneficiary to be different to funder)
      * @param _allowBeneficialInvestments Boolean to allow or disallow beneficial investments
      */
-    function changeAllowBeneficialInvestments(bool _allowBeneficialInvestments) public onlyOwner {
+    function changeAllowBeneficialInvestments(bool _allowBeneficialInvestments) external onlyOwner {
         require(_allowBeneficialInvestments != allowBeneficialInvestments, "Value unchanged");
         allowBeneficialInvestments = _allowBeneficialInvestments;
         emit SetAllowBeneficialInvestments(allowBeneficialInvestments);
@@ -508,7 +508,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
         uint256 _investmentValue,
         FundRaiseType _fundRaiseType
     )
-        public
+        external
         view
         returns(uint256 spentUSD, uint256 spentValue, uint256 tokensMinted)
     {
@@ -732,7 +732,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @param _amount Value to convert to USD
      * @return uint256 Value in USD
      */
-    function convertToUSD(FundRaiseType _fundRaiseType, uint256 _amount) public view returns(uint256) {
+    function convertToUSD(FundRaiseType _fundRaiseType, uint256 _amount) external view returns(uint256) {
         uint256 rate = getRate(_fundRaiseType);
         return DecimalMath.mul(_amount, rate);
     }
@@ -743,7 +743,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @param _amount Value to convert from USD
      * @return uint256 Value in ETH or POLY
      */
-    function convertFromUSD(FundRaiseType _fundRaiseType, uint256 _amount) public view returns(uint256) {
+    function convertFromUSD(FundRaiseType _fundRaiseType, uint256 _amount) external view returns(uint256) {
         uint256 rate = getRate(_fundRaiseType);
         return DecimalMath.div(_amount, rate);
     }
@@ -776,7 +776,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * param _fundRaiseType The fund raising currency (e.g. ETH, POLY, SC) to calculate sold tokens for
      * @return uint256 Total number of tokens sold for ETH
      */
-    function getTokensSoldFor(FundRaiseType _fundRaiseType) public view returns (uint256) {
+    function getTokensSoldFor(FundRaiseType _fundRaiseType) external view returns (uint256) {
         uint256 tokensSold;
         for (uint256 i = 0; i < tiers.length; i++) {
             tokensSold = tokensSold.add(tiers[i].minted[uint8(_fundRaiseType)]);
@@ -789,7 +789,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * param _tier The tier to return minted tokens for
      * @return uint256[] array of minted tokens in each fund raise type
      */
-    function getTokensMintedByTier(uint256 _tier) public view returns (uint256[]) {
+    function getTokensMintedByTier(uint256 _tier) external view returns (uint256[]) {
         require(_tier < tiers.length, "Invalid tier");
         uint256[] memory tokensMinted = new uint256[](3);
         tokensMinted[0] = tiers[_tier].minted[uint8(FundRaiseType.ETH)];
@@ -803,7 +803,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * param _tier The tier to calculate sold tokens for
      * @return uint256 Total number of tokens sold in the tier
      */
-    function getTokensSoldByTier(uint256 _tier) public view returns (uint256) {
+    function getTokensSoldByTier(uint256 _tier) external view returns (uint256) {
         require(_tier < tiers.length, "Incorrect tier");
         uint256 tokensSold;
         tokensSold = tokensSold.add(tiers[_tier].minted[uint8(FundRaiseType.ETH)]);
@@ -816,7 +816,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @notice Return the total no. of tiers
      * @return uint256 Total number of tiers
      */
-    function getNumberOfTiers() public view returns (uint256) {
+    function getNumberOfTiers() external view returns (uint256) {
         return tiers.length;
     }
 
@@ -824,7 +824,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @notice Return the usd tokens accepted by the STO
      * @return address[] usd tokens
      */
-    function getUsdTokens() public view returns (address[]) {
+    function getUsdTokens() external view returns (address[]) {
         return usdTokens;
     }
 
@@ -848,7 +848,7 @@ contract USDTieredSTO is USDTieredSTOStorage, ISTO, ReentrancyGuard {
      * @return Amount of tokens sold.
      * @return Array of bools to show if funding is allowed in ETH, POLY, SC respectively
      */
-    function getSTODetails() public view returns(uint256, uint256, uint256, uint256[], uint256[], uint256, uint256, uint256, bool[]) {
+    function getSTODetails() external view returns(uint256, uint256, uint256, uint256[], uint256[], uint256, uint256, uint256, bool[]) {
         uint256[] memory cap = new uint256[](tiers.length);
         uint256[] memory rate = new uint256[](tiers.length);
         for(uint256 i = 0; i < tiers.length; i++) {
diff --git a/contracts/proxy/USDTieredSTOProxy.sol b/contracts/proxy/USDTieredSTOProxy.sol
index a412a8278..050e14a21 100644
--- a/contracts/proxy/USDTieredSTOProxy.sol
+++ b/contracts/proxy/USDTieredSTOProxy.sol
@@ -4,13 +4,13 @@ import "../storage/USDTieredSTOStorage.sol";
 import "./OwnedProxy.sol";
 import "../Pausable.sol";
 import "openzeppelin-solidity/contracts/ReentrancyGuard.sol";
-import "../modules/STO/ISTOStorage.sol";
+import "../modules/STO/STOStorage.sol";
 import "../modules/ModuleStorage.sol";
 
 /**
  * @title USDTiered STO module Proxy
  */
-contract USDTieredSTOProxy is USDTieredSTOStorage, ISTOStorage, ModuleStorage, Pausable, ReentrancyGuard, OwnedProxy {
+contract USDTieredSTOProxy is USDTieredSTOStorage, STOStorage, ModuleStorage, Pausable, ReentrancyGuard, OwnedProxy {
 
     /**
     * @notice Constructor
diff --git a/contracts/storage/USDTieredSTOStorage.sol b/contracts/storage/USDTieredSTOStorage.sol
index f801000e0..31e9d803d 100644
--- a/contracts/storage/USDTieredSTOStorage.sol
+++ b/contracts/storage/USDTieredSTOStorage.sol
@@ -51,9 +51,6 @@ contract USDTieredSTOStorage {
     // Whether or not the STO has been finalized
     bool public isFinalized;
 
-    // Address where ETH, POLY & Stable Coin funds are delivered
-    address public wallet;
-
     // Address of issuer reserve wallet for unsold tokens
     address public reserveWallet;
 
diff --git a/test/p_usd_tiered_sto.js b/test/p_usd_tiered_sto.js
index ae1fbbbb0..fca953a18 100644
--- a/test/p_usd_tiered_sto.js
+++ b/test/p_usd_tiered_sto.js
@@ -1246,7 +1246,8 @@ contract("USDTieredSTO", accounts => {
             await I_USDTieredSTO_Array[stoId].buyWithUSD(NONACCREDITED1, investment_DAI, I_DaiToken.address, { from: NONACCREDITED1 });
 
             // Change Stable coin address
-            await I_USDTieredSTO_Array[stoId].modifyAddresses(WALLET, RESERVEWALLET, [I_PolyToken.address], { from: ISSUER });
+            let I_DaiToken2 = await PolyTokenFaucet.new({from: POLYMATH});
+            await I_USDTieredSTO_Array[stoId].modifyAddresses(WALLET, RESERVEWALLET, [I_DaiToken2.address], { from: ISSUER });
 
             // NONACCREDITED DAI
             await catchRevert(I_USDTieredSTO_Array[stoId].buyWithUSD(NONACCREDITED1, investment_DAI, I_DaiToken.address, { from: NONACCREDITED1 }));