There are available ADF's that can process both sides of a document: these are known as two-sided or duplex ADF's. These are further classified as:
- Reversing Automatic Document Feeders (RADF): these scan one side of a page, then flip it and scan the other side.
- Duplexing Automatic Document Feeders (DADF): these scan both sides in one pass. The advantage of the DADF is faster speed for two-sided originals.
These devices are available in all the machines that process documents; naturally they are more expensive than the one-sided ADF's; the duplexing ADF's are more costly than the reversing ADF's but are much faster and are orders of magnitude more reliable.
The design and operation of these devices don't need much explanation. The reversing ADF's can function in two ways: the page can be flipped immediately, making it a one-page-at-a-time process, or a certain number of pages can be scanned on one side and placed in a magazine somewhere, which is flipped over and the documents are then scanned on the other side. (In fact, copiers use this method for two-sided copying: but they have to have two internal magazines, one for the source documents and one for the output documents). The immediate-flip process has one great advantage (which may or may not overcome its obvious speed limitation) in that the two sides of the document always stay together after being scanned; the magazine-style system leads to problems in properly collating the copies, which the scanner software may or may not be able to handle. If all else fails, then an acceptable way to handle this to do two-sided to one-sided copying.
If an auxiliary input slot is provided (i.e., manual ADF) for the magazine-style reversing ADF, there is an obvious and baffling problem to be faced if the placement of the manual sheet is important.
Contrariwise, the duplexing systems involve no special handling of the document: instead, they have two reader devices, one for each side of the document. They have no trouble keeping pages in the right order, as the images are always available together. But unless the control panel and software are very sophisticated, a manual sheet will always be placed after both sides of the original have been processed.
Problems such as these, involving logical rather than physical issues, can be overcome to a greater or lesser extent by the use of appropriate software. Scanned documents can be brought into the proper sequence by laborious cutting & pasting; pages inserted manually can be re-paginated easily; and so forth. For high-volume users, a duplex scanner will eliminate the collating problem and over the long run be more cost-effective than relying on word-processing or page-making software.
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