EET 405 - Advanced Digital

Parallel Ports

Parallel Ports

In contrast to serial ports, parallel ports ‘present’ all bits at one time.

‘The parallel port reflects a hardware engineer’s concept of the way in which communication should work.’

Eight Data bits are represented by 8 wires– Voltage levels are standard TTL - 0/5v

Parallel Ports

Flow control is provided by hardware signals– Handshaking lines

Original Parallel Ports were single direction (output only)

Modern ports allow for bi-directional communications

Parallel Ports

Three ranges of I/O ports are assigned.– 03BCh, 0378h, 0278h.– LPT1, LPT2, LPT3

Interrupts provided – 07h and 05h

Centronics Connectors (25pin)

On 25 pin connector - IEEE 1284– 1 - strobe– 2-9 - data bits 0-7– 10 - Acknowledge– 11 - Busy– 12 - Paper Out– 13 - Select– 14 - Auto LF

– 15 - error– 16 - Printer Initialize– 17 - Select inpu– 18-25 Ground

Parallel Port Signals

Data lines - maintain the bit through the transfer period

Strobe Line– negative pulse - print character– 1 microsecond long

Busy– high = Printer is busy

Acknowledge Line– normally high - strobes low to signal that

printer is ready Select

– high = printer is on line Paper Empty

– high = printer is out of paper

Fault (error)– high = printer has some problem not identified

above Initialize Printer

– low makes printer reset itself Select Input

– low = printer should select itself

Autofeed– low = automatically send LF after DR

Bi-directional Parallel Ports

Dependent on the configuration of the PPI chip used

can accept 8 bit data on lines Bus Mastering was added to allow DMA

transfer to/from devices connected to parallel ports.

Enhanced Parallel Ports

EPP - a faster port that has a streamlined logical interface and explicit definition of electrical parameters

Standard PP uses three input/output ports for control and moving data -- EPP uses 8

base + 3 = EPP address port – Signals that the data bits now contain an

address

EPP

base + 4 to base +7 = EPP Data buffer– allows host computer to write 32 bit double

word to all 4 buffers simultaneously the 25 pin connector is assigned as a

standard parallel port When Enhanced mode is enabled - only 8

data and 5 signals are used.

EPP signals

Write - pin 1 (low = write is occuring) Data Strobe - pin 14 (data valid) Address Strobe - pn 17 (address info is

available) Wait - pin 11 (busy) Interrupt - pin 10 (high = request interrupt) the other four ports are used as normal

EPP CABLE

due to the high speed nature of the port, the cable is specifically engineered to reduce noise.

Double-shielded ~ 2 Mbytes per second

– can expect to increase to 8 Mbytes /sec.

Extended Capabilities Ports ECP

adds 2 modes– fast two-way communication mode– data compression (RLE)

Allows for 128 addresses Additional control registers at 0400h higher

than the base registers 0402h - Extended control register - sets mode

Modes of ECP Table 19-12Name Address Mode Function

Data Base PC, PS/2 Data Reg

ecpAFifo Base ECP EDP FIFO buffer

DSR Base+1 All Status Register

DCR Base+2 All Control Register

cFifo Base+400 EPP EPP FIFO (data) Buffer

ecpDFifo Base+400 ECP ECP FIFO (data) Buffer

tFifo Base+400 Test Test FIFO

cnfgA Base+400 Config Config. Register A

cnfgB Base+401 Config Config. Register B

ecr Base+402 All Extended control reg.

backwards compatible in ECP mode - can write 8, 16, or 32 bits

wide. Allows up to 128 devices on a single port

Speeds

conventional port up to 250,000 bytes per second– with overhead expect 80-90kbps

EPP can produce approx. 800 kbps– can expect up to 8 mbps in the future

ECP incorporates compression – Run Length Encoding– sends byte + multiplier

Assignment