Dosadi does not offer support for EZTwain Classic. For support and greatly expanded features, see our EZTwain Pro library.
Here is an untested semi-manual translation of the EZTwain Classic declarations, into Clarion.
We are willing to correct and/or update this information - send comments to: support@dosadi.com.

Section('*** Type Equates ***')
! equate these data types so the code has more meaning then
! just seeing an UNSIGNED everywhere
LPVOID EQUATE(UNSIGNED) !Pointer to something
HANDLE EQUATE(UNSIGNED) !Handle in general
HWND EQUATE(HANDLE) !Window Handle
HDIB EQUATE(HANDLE) !DIB Handle
HPALETTE EQUATE(HANDLE)
HDC EQUATE(HANDLE)
HFILE EQUATE(HANDLE)
HBITMAP EQUATE(HANDLE)

Section('*** Equates ***')

! Pixel type masks, for TWAIN_AcquireNative:
TWAIN_BW EQUATE(1)
TWAIN_GRAY EQUATE(2)
TWAIN_RGB EQUATE(4)
TWAIN_PALETTE EQUATE(8)
TWAIN_CMY EQUATE(16)
TWAIN_CMYK EQUATE(32)
TWAIN_ANYTYPE EQUATE(0)
! TWAIN state:
TWAIN_PRESESSION EQUATE(1)
TWAIN_SM_LOADED EQUATE(2)
TWAIN_SM_OPEN EQUATE(3)
TWAIN_SOURCE_OPEN EQUATE(4)
TWAIN_SOURCE_ENABLED EQUATE(5)
TWAIN_TRANSFER_READY EQUATE(6)
TWAIN_TRANSFERRING EQUATE(7)
! Transfer Modes (see TWAIN_SetXferMech)
XFERMECH_NATIVE EQUATE(0)
XFERMECH_FILE EQUATE(1)
XFERMECH_MEMORY EQUATE(2)

Section('*** Map ***')
MAP
MODULE('Eztw32.dll')

TWAIN_AcquireNative(HWND hwndApp, UNSIGNED wPixTypes),HDIB,PASCAL,RAW,DLL(1)
! The minimal use of EZTWAIN.DLL is to just call this routine, with 0 for
! both params. EZTWAIN creates a window if hwndApp is 0.
!
! Acquires a single image, from the currently selected Data Source, using
! Native-mode transfer. It waits until the source closes (if it's modal) or
! forces the source closed if not. The return value is a handle to the
! acquired image. Only one image can be acquired per call.
!
! Under Windows, the return value is a global memory handle - applying
! GlobalLock to it will return a (huge) pointer to the DIB, which
! starts with a BITMAPINFOHEADER.
! NOTE: You are responsible for disposing of the returned DIB - these things
! can eat up your Windows memory fast! See TWAIN_FreeNative below.
!
! The image type can be restricted using the following masks. A mask of 0
! means 'any pixel type is welcome'.
! Caution: You should not assume that the source will honor a pixel type
! restriction! If you care, check the parameters of the DIB.

TWAIN_FreeNative(HDIB hdib),PASCAL,RAW,DLL(1)
! Release the memory allocated to a native format image, as returned by
! TWAIN_AcquireNative. (If you are coding in C or C++, this is just a call
! to GlobalFree.)
! If you use TWAIN_AcquireNative and don't free the returned image handle,
! it stays around taking up Windows (virtual) memory until your application
! terminates.

TWAIN_AcquireToClipboard(HWND hwndApp, UNSIGNED wPixTypes),BOOL,PASCAL,RAW,DLL(1)
TWAIN_AcquireToFilename(HWND hwndApp, *CSTRING sFile),SIGNED,PASCAL,RAW,DLL(1)
! Acquire an image and write it to a .BMP (Windows Bitmap) file.
! The file name and path in pszFile are used. If pszFile is NULL or
! points to an empty string, the user is prompted with a Save File dialog.
! Return values:
! 0 success
! -1 Acquire failed OR user cancelled File Save dialog
! -2 file open error (invalid path or name, or access denied)
! -3 (weird) unable to lock DIB - probably an invalid handle.
! -4 writing BMP data failed, possibly output device is full

TWAIN_SelectImageSource(HWND hwnd),BOOL,PASCAL,RAW,DLL(1)
! Prompts the user to select the system-wide Default TWAIN device.
! EZTwain always acquires images from the default TWAIN device.
!
! Note: If only one TWAIN device is installed on a system, it is selected
! automatically, so there is no need for the user to do Select Source.
! You should not require your users to do Select Source before Acquire.
!
! This function posts the Source Manager's Select Source dialog box.
! It returns after the user either OK's or CANCEL's that dialog.
! A return of 1 indicates OK, 0 indicates one of the following:
! a) The user cancelled the dialog
! b) The Source Manager found no data sources installed
! c) There was a failure before the Select Source dialog could be posted

TWAIN_IsAvailable(),BOOL,PASCAL,RAW,DLL(1)
! Call this function any time to find out if TWAIN is installed on the
! system. It takes a little time on the first call, after that it's fast,
! just testing a flag. It returns 1 if the TWAIN Source Manager is
! installed & can be loaded, 0 otherwise.

TWAIN_EasyVersion(),SIGNED,PASCAL,RAW,DLL(1)
! Returns the version number of EZTWAIN.DLL, multiplied by 100.
! So e.g. version 1.13 will return 113 from this call.

TWAIN_State(),SIGNED,PASCAL,RAW,DLL(1)
! Returns the TWAIN Protocol State per the spec.

TWAIN_DibDepth(HDIB hdib),SIGNED,PASCAL,RAW,DLL(1)
TWAIN_DibWidth(HDIB hdib),SIGNED,PASCAL,RAW,DLL(1)
TWAIN_DibHeight(HDIB hdib),SIGNED,PASCAL,RAW,DLL(1)
TWAIN_DibNumColors(HDIB hdib),SIGNED,PASCAL,RAW,DLL(1)
TWAIN_RowSize(HDIB hdib),SIGNED,PASCAL,RAW,DLL(1)
TWAIN_ReadRow(HDIB hdib, SIGNED n, *BYTE prow),PASCAL,RAW,DLL(1)
TWAIN_CreateDibPalette(HDIB hdib),HPALETTE,PASCAL,RAW,DLL(1)
TWAIN_DrawDibToDC(HDC hDC, SIGNED dx, SIGNED dy, SIGNED w, SIGNED h, HDIB hdib, SIGNED sx, SIGNED sy),PASCAL,RAW,DLL(1)

TWAIN_WriteNativeToFilename(HDIB hdib, *CSTRING sFile),SIGNED,PASCAL,RAW,DLL(1)
! Writes a DIB handle to a .BMP file
!
! hdib = DIB handle, as returned by TWAIN_AcquireNative
! pszFile = far pointer to NUL-terminated filename
! If pszFile is NULL or points to a null string, prompts the user
! for the filename with a standard file-save dialog.
!
! Return values:
! 0 success
! -1 user cancelled File Save dialog
! -2 file open error (invalid path or name, or access denied)
! -3 (weird) unable to lock DIB - probably an invalid handle.
! -4 writing BMP data failed, possibly output device is full

TWAIN_LoadNativeFromFilename(*CSTRING sFile),HDIB,PASCAL,RAW,DLL(1)
! Load a .BMP file and return a DIB handle (as from AcquireNative.)
! Accepts a filename (including path & extension).
! If pszFile is NULL or points to a null string, the user is prompted.
! Returns a DIB handle if successful, otherwise NULL.

TWAIN_SetHideUI(BOOL bHide),PASCAL,RAW,DLL(1)
TWAIN_GetHideUI(),BOOL,PASCAL,RAW,DLL(1)
! These functions control the 'hide source user interface' flag.
! This flag is cleared initially, but if you set it non-zero, then when
! a device is enabled it will be asked to hide its user interface.
! Note that this is only a request - some devices will ignore it!
! This affects AcquireNative, AcquireToClipboard, and EnableSource.
! If the user interface is hidden, you will probably want to set at least
! some of the basic acquisition parameters yourself - see
! SetUnits, SetPixelType, SetBitDepth and SetResolution below.
! See also: HasControllableUI

TWAIN_RegisterApp(SIGNED nMajorNum, SIGNED nMinorNum, SIGNED nLanguage, |
SIGNED nCountry, *CSTRING sVersion, *CSTRING sMfg, *CSTRING sFamily, |
*CSTRING sAppTitle),PASCAL,RAW,DLL(1)

! nMajorNum major and incremental revision of application.
! nMinorNum e.g. version 4.5: nMajorNum = 4, nMinorNum = 5
! nLanguage (human) language (use TWLG_xxx from TWAIN.H)
! nCountry country (use TWCY_xxx from TWAIN.H)
! lpszVersion version info string e.g. "1.0b3 Beta release"
! lpszMfg name of mfg/developer e.g. "Crazbat Software"
! lpszFamily product family e.g. "BitStomper"
! lpszProduct specific product e.g. "BitStomper Deluxe Pro"
! TWAIN_RegisterApp can be called *AS THE FIRST CALL*, to register the
! application. If this function is not called, the application is given a
! 'generic' registration by EZTWAIN.
! Registration only provides this information to the Source Manager and any
! sources you may open - it is used for debugging, and (frankly) by some
! sources to give special treatment to certain applications.

TWAIN_GetResultCode(),UNSIGNED,PASCAL,RAW,DLL(1)
! Return the result code (TWRC_xxx) from the last triplet sent to TWAIN

TWAIN_GetConditionCode(),UNSIGNED,PASCAL,RAW,DLL(1)
! Return the condition code from the last triplet sent to TWAIN.
! (To be precise, from the last call to TWAIN_DS below)
! If a source is NOT open, return the condition code of the source manager.

TWAIN_ErrorBox(*CSTRING sMsg),PASCAL,RAW,DLL(1)
! Post an error message dialog with an exclamation mark, OK button,
! and the title 'TWAIN Error'.
! pszMsg points to a null-terminated message string.

TWAIN_ReportLastError(*CSTRING sMsg),PASCAL,RAW,DLL(1)
! Like TWAIN_ErrorBox, but if some details are available from
! TWAIN about the last failure, they are included in the message box.

TWAIN_LoadSourceManager(),BOOL,PASCAL,RAW,DLL(1)
! Finds and loads the Data Source Manager, TWAIN.DLL.
! If Source Manager is already loaded, does nothing and returns TRUE.
! This can fail if TWAIN.DLL is not installed (in the right place), or
! if the library cannot load for some reason (insufficient memory?) or
! if TWAIN.DLL has been corrupted.

TWAIN_OpenSourceManager(HWND hwnd),BOOL,PASCAL,RAW,DLL(1)
! Opens the Data Source Manager, if not already open.
! If the Source Manager is already open, does nothing and returns TRUE.
! This call will fail if the Source Manager is not loaded.

TWAIN_OpenDefaultSource(),BOOL,PASCAL,RAW,DLL(1)
! This opens the source selected in the Select Source dialog.
! If a source is already open, does nothing and returns TRUE.
! Fails if the source manager is not loaded and open.

TWAIN_EnableSource(HWND hwnd),BOOL,PASCAL,RAW,DLL(1)
! Enables the open Data Source. This posts the source's user interface
! and allows image acquisition to begin. If the source is already enabled,
! this call does nothing and returns TRUE.

TWAIN_DisableSource(),BOOL,PASCAL,RAW,DLL(1)
! Disables the open Data Source, if any.
! This closes the source's user interface.
! If there is not an enabled source, does nothing and returns TRUE.

TWAIN_CloseSource(),BOOL,PASCAL,RAW,DLL(1)
! Closes the open Data Source, if any.
! If the source is enabled, disables it first.
! If there is not an open source, does nothing and returns TRUE.

TWAIN_CloseSourceManager(HWND hwnd),BOOL,PASCAL,RAW,DLL(1)
! Closes the Data Source Manager, if it is open.
! If a source is open, disables and closes it as needed.
! If the Source Manager is not open, does nothing and returns TRUE.

TWAIN_UnloadSourceManager(),BOOL,PASCAL,RAW,DLL(1)
! Unloads the Data Source Manager i.e. TWAIN.DLL - releasing
! any associated memory or resources.
! This call will fail if the Source Manager is open, otherwise
! it always succeeds and returns TRUE.

TWAIN_EndXfer(),BOOL,PASCAL,RAW,DLL(1)
TWAIN_AbortAllPendingXfers(),BOOL,PASCAL,RAW,DLL(1)

!--------- High-level Capability Negotiation Functions --------

! These functions should only be called in State 4 (TWAIN_SOURCE_OPEN)

TWAIN_NegotiateXferCount(SIGNED nXfers),BOOL,PASCAL,RAW,DLL(1)
! Negotiate with open Source the number of images application will accept.
! This is only allowed in State 4 (TWAIN_SOURCE_OPEN)
! nXfers = -1 means any number

TWAIN_NegotiatePixelTypes(UNSIGNED wPixTypes),BOOL,PASCAL,RAW,DLL(1)
! Negotiate with the source to restrict pixel types that can be acquired.
! This tries to restrict the source to a *set* of pixel types,
! See TWAIN_AcquireNative above for some mask constants.
! --> This is only allowed in State 4 (TWAIN_SOURCE_OPEN)
! A parameter of 0 (TWAIN_ANYTYPE) causes no negotiation & no restriction.
! You should not assume that the source will honor your restrictions, even
! if this call succeeds!

TWAIN_GetCurrentUnits(),SIGNED,PASCAL,RAW,DLL(1)
! Ask the source what its current unit of measure is.
! If anything goes wrong, this function just returns TWUN_INCHES (0).

TWAIN_SetCurrentUnits(SIGNED nUnits),BOOL,PASCAL,RAW,DLL(1)
! Set the current unit of measure for the source.
! Unit of measure codes are in TWAIN.H, but TWUN_INCHES is 0.

TWAIN_GetBitDepth(),SIGNED,PASCAL,RAW,DLL(1)
! Get the current bitdepth, which can depend on the current PixelType.
! Bit depth is per color channel e.g. 24-bit RGB has bit depth 8.
! If anything goes wrong, this function returns 0.

TWAIN_SetBitDepth(SIGNED nBits),BOOL,PASCAL,RAW,DLL(1)
! (Try to) set the current bitdepth (for the current pixel type).

TWAIN_GetPixelType(),SIGNED,PASCAL,RAW,DLL(1)
! Ask the source for the current pixel type.
! If anything goes wrong (it shouldn't), this function returns 0 (TWPT_BW).

TWAIN_SetCurrentPixelType(SIGNED nPixType),BOOL,PASCAL,RAW,DLL(1)
! (Try to) set the current pixel type for acquisition.
! This is only allowed in State 4 (TWAIN_SOURCE_OPEN)
! The source may select this pixel type, but don't assume it will.

TWAIN_GetCurrentResolution(),REAL,PASCAL,RAW,DLL(1)
! Ask the source for the current (horizontal) resolution.
! Resolution is in dots per current unit! (See TWAIN_GetCurrentUnits above)
! If anything goes wrong (it shouldn't) this function returns 0.0

TWAIN_GetYResolution(),REAL,PASCAL,RAW,DLL(1)
! Returns the current vertical resolution, in dots per *current unit*.
! In the event of failure, returns 0.0.

TWAIN_SetCurrentResolution(REAL dRes),BOOL,PASCAL,RAW,DLL(1)
! (Try to) set the current resolution for acquisition.
! Resolution is in dots per current unit! (See TWAIN_GetCurrentUnits above)
! This is only allowed in State 4 (TWAIN_SOURCE_OPEN)
! Note: The source may select this resolution, but don't assume it will.

TWAIN_SetContrast(REAL dCon),BOOL,PASCAL,RAW,DLL(1)
! (Try to) set the current contrast for acquisition.
! The TWAIN standard says that the range for this cap is -1000 ... +1000

TWAIN_SetBrightness(REAL dBri),BOOL,PASCAL,RAW,DLL(1)
! (Try to) set the current brightness for acquisition.
! The TWAIN standard says that the range for this cap is -1000 ... +1000

TWAIN_SetXferMech(SIGNED mech),BOOL,PASCAL,RAW,DLL(1)
TWAIN_XferMech(),SIGNED,PASCAL,RAW,DLL(1)
! (Try to) set or get the transfer mode - one of XFERMECH_NATIVE, XFERMECH_FILE, XFERMECH_MEMORY 2

!--------- Low-level Capability Negotiation Functions --------

! Setting a capability is valid only in State 4 (TWAIN_SOURCE_OPEN)
! Getting a capability is valid in State 4 or any higher state.

TWAIN_SetCapOneValue(UNSIGNED Cap, UNSIGNED ItemType, SIGNED ItemVal),BOOL,PASCAL,RAW,DLL(1)
! Do a DAT_CAPABILITY/MSG_SET, on capability 'Cap' (e.g. ICAP_PIXELTYPE,
! CAP_AUTOFEED, etc.) using a TW_ONEVALUE container with the given item type
! and value. The item value must fit into 32 bits.
! Returns TRUE (1) if successful, FALSE (0) otherwise.

TWAIN_GetCapCurrent(UNSIGNED Cap, UNSIGNED ItemType, *BYTE pVal),BOOL,PASCAL,RAW,DLL(1)
! Do a DAT_CAPABILITY/MSG_GETCURRENT on capability 'Cap'.
! Copy the current value out of the returned container into *pVal.
! If the operation fails (the source refuses the request), or if the
! container is not a ONEVALUE or ENUMERATION, or if the item type of the
! returned container is incompatible with the expected TWTY_ type in nType,
! returns FALSE. If this function returns FALSE, *pVal is not touched.

TWAIN_ToFix32(REAL d),SIGNED,PASCAL,RAW,DLL(1)
! Convert a floating-point value to a 32-bit TW_FIX32 value that can be passed
! to e.g. TWAIN_SetCapOneValue

TWAIN_Fix32ToFloat(SIGNED nfix),REAL,PASCAL,RAW,DLL(1)
! Convert a TW_FIX32 value (as returned from some capability inquiries)
! to a double (floating point) value.