Input/Output devices Part 1: Parallel, serial, disk and tape controllers

1. Input/Output devicesPart 1: Parallel, serial, disk and tape controllers dr.ir. A.C. VerschuerenEindhoven University of TechnologySection of Digital Information Systems

2. What will we talk about ? • A subset of available input/output controllers • Basic and extended parallel I/O • Different types of serial I/O, including LAN’s • Disk controllers • Display controllers of different complexity • Programmable I/O controllers • More is available, but only mentioned here • Analog  digital conversion • Data compression/expansion, encryption/decryption • Motor controllers

3. Parallel I/O: more bits at the same time • In principle faster than 1 bit at a time serial I/O • Needs multiple physically separate signal paths • Cabling and send/receive electronics are expensive ! • Use low(er) cost simple electronics and cables • Parallel I/O is generally only used for short distances Actual speed not much higher than serial I/O !

4. data available busy request acknowledge PROC DEVICE acknowl. PROC strobe DEVICE data data 'dav' busy ‘req’ 'ack' strobe ‘ack’ data valid data valid Processor initiates transferbyindicating 'data available' Device initiates transferby indicating 'not busy' Device initiates transferby indicating ’request' Parallel I/O synchronisation • Different sets of 'handshake' signals in use For input handshake:‘PROC’  ‘DEVICE’

5. dav out dav out ack out ack out dav in dav in PROC DEVICE ack in ack in data valid out valid in data Bidirectional parallel input/output • Extra ‘rules’ needed to prevent problems • 'Never output data while the ack output is active' Higher level protocol needed to switch direction

6. 'device under test' computer power supply frequency meter 20.000 on/off/voltage readings GPIB Multi-directional parallel I/O buses (1) • 'General Purpose Interface Bus' for laboratories 20 MHz • Devices function as 'talkers', 'listeners' and 'controllers' • Very complex bus protocol (> 1 controller allowed) • Max. distance 20 metres, up to 500 KiloBytes/second

7. Multi-directional parallel I/O buses (2) • The 'Small Computer Systems Interface' (SCSI) • A (byte-parallel) connection between computers and tape/disk storage devices, scanners, printers... • Up to 8 devices can be controlled at the same time • Speed > 1.5 MegaByte/second, distance < 10 metres • 'SCSI' is of a family of 'standard' interfaces  • For all kinds of parallel input/output interfaces, specialised interface components exist !

8. Serial I/O: one bit at a time • Handshake signals merged with actual data bits • Number of physical connections reduced to minimum • High quality cable and send/receive electronics • Greater distances can be handled (even worldwide) • High bit rates possible (fiber: > 1 GigaBits/second !) Parallel I/O signal ‘timing skew’ problem eliminated • Multi-directional communication possible • A 'Local Area Network' is a complex serial I/O protocol

9. Problems to solve for serial I/O 1)Where are the boundaries between the bits ? • We need to know where the real data bits start and end in order to communicate 2)Where are the boundaries between the words ? • Processors are generally word-oriented and must know which bits belong to the same word 3)Which words belong together ? • This information is vital if several communicating partners make use of the same channel (LAN’s !)

10. ‘stop bit’(s) ‘start bit' data bits line signal bit 0 bit 1 bit 2 bit 6 bit 7 t 2t 2t 2t N x 2t 2t 2t Asynchronous serial I/O • Each word is sent separately The start transition is not synchronised: 'asynchronous' • Bit positions determined by fixed time delays The transmitter’s time ‘t’ must be known by the receiver!

11. Remarks on asynchronous serial I/O • In its basic form very simple to implementstandard on most single-chip micro-computers • Error detection can be provided by transmitting a 'parity' bit (XOR of data bits) after last data bit • No protocols for grouping words or handshaking • Some ASCII control codes (mis)used for these • 'DC1'/'DC3' and 'ACK'/'NAK' for flow control • 'STX'/'ETX' for grouping (‘start of text’ / ‘end of text’) ‘XON’/’XOFF’

12. Synchronous serial I/O: more efficient • Bit boundary detection synchronises on each line transition, two ‘tricks’ ensure enough transitions • 'Non Return to Zero Invert': a '1' bit keeps the line state stable, a '0' bit lets the line make a transition • ’Bit stuffing': after 5 consecutive '1' bits, the transmitter inserts a '0' bit (which is removed again by the receiver) • Data words are always grouped in 'frames' • Frames start and end with a 'flag': the fixed bit pattern '01111110’ (cannot be data through the bit stuffing !) NO start and stop bits needed !

13. Synchronous serial I/O protocol family • The presence of frames makes the synchronous serial I/O protocol very powerful 'HDLC', 'SDLC' and 'X-25' are family members • First bytes of a frame contain address and control info • A 'Cyclic Redundancy Check’ is used to detect errors Remainder of dividing frame contents by a fixed value • Basic synchronous serial I/O is rather simple to implement, but a complete protocol is very complex !

14. Disk and tape controllers • Data is stored as sequential streams of bits:a specialised form of serial I/O • Controllers also need logic to position the read/write heads and control disk/tape movement • Magnetic/optical media do not allow continuous streams of '0' or '1' bits to be stored directly • Encoding must be used to ensure enough transitions • 'Bit stuffing' is impossible because it removes the direct relation between the number of bits stored and written !

15. '1' '0' '0' '1' '0' '1' FM: MFM: 'cell' Disk/tape data modulation (1) • ‘Frequency Modulation’ makes a transition at each bit ‘cell’ boundary and halfway a '1' bit • ‘Modified Frequency Modulation’ transitions halfway a '1' bit and between two '0' bits • This modulation method has problems with repeated'110'patterns...

16. Disk/tape data modulation (2) • ‘Run Length Limited’ encodes groups of 4 bits in 5 cells with transitions only at cell boundaries • The length of a 'run' of consecutive '0' or '1' cells is limited by the chosen encoding • All modulation techniques try to use as few transitions as possible for each bit • Limit the frequency bandwidth and lower the highest frequency in the signalthis simplifies the analog parts of the system!

17. written during 'format' rewritten on each data block write 'index mark' CRC or 'Error Correcting Code' 'data mark' position info: head/track/sector or track/block actual data block: 256, 512, 1024... bytes Disk and tape data formats • Data is stored in fixed-size data ‘fields’,each preceeded by a separate control field crc • The 'marks' are a kind of ‘start of frame’ flags • Position info is checked before actual writing is done !

18. Floppy and hard disk controllers • These are standard (‘off the shelf’) items • Tape drives use disk-like formats and interfaces ! • They differ in how they handle multiple drives • Read/write one drive while 'seeking' on others ? • They differ in their DMA capabilities • One block to several tracks with a single command • Separate commands to command list driven Complex ones are complete disk/tape coprocessors!