Mostexternal devices are capable of both input and output (I/O). Some devices areinherently input-only (also called read-only) or inherently output-only (alsocalled write-only). Regardless of whether a device is I/O, read-only, orwrite-only, external devices can be classified as block or character devices.
Acharacter device is one that inputs or outputs data in a stream ofcharacters, bytes, or bits. Character devices can further be classified asserial or parallel. Examples of character devices include printers, keyboards,and mice.
Aserial device streams data as a series of bits, moving data one bit at atime. Examples of serial devices include printers and MODEMs.
Aparallel device streams data in a small group of bits simultaneously.Usually the group is a single eight-bit byte (or possibly seven or nine bits,with the possibility of various control or parity bits included in the datastream). Each group usually corresponds to a single character of data. Rarelythere will be a larger group of bits (word, longword, doubleword, etc.). Themost common parallel device is a printer (although most modern printers haveboth a serial and a parallel connection, allowing greater connectionflexibility).
Ablock device moves large blocks of data at once. This may be physicallyimplemented as a serial or parallel stream of data, but the entire block getstransferred as single packet of data. Most block devices are random access(that is, information can be read or written from blocks anywhere on thedevice). Examples of random access block devices include hard disks, floppydisks, and drum drives. Examples of sequential access block devcies includemagnetic tape drives and high speed paper tape readers.