System for transferring data in parallel to host computer using both of the rising and falling edges of host busy signals as transfer instruction signals

摘要

An image scanner 30 is provided with a pair of buffers 54 and 55 which have the same amount of storage area of eight bytes. Except for the first writing operation to write the first eight bytes to the first buffer 54, writing operations and transferring operations are performed simultaneously. That is, data is written into the first buffer 54 while data is retrieved from the second buffer 55, and data is written into the second buffer 55 while data is retrieved from the first buffer 54. Data is thus retrieved from the buffers 54 and 55 in alternation and transferred to the host computer 10.