EET Advanced Digital Parallel Ports

n In contrast to serial ports, parallel ports ‘present’ all bits at one time. n ‘The parallel port reflects a hardware engineer’s concept of the way in which communication should work.’ n Eight Data bits are represented by 8 wires – Voltage levels are standard TTL - 0/5v

Parallel Ports n Flow control is provided by hardware signals – Handshaking lines n Original Parallel Ports were single direction (output only) n Modern ports allow for bi-directional communications

Parallel Ports n Three ranges of I/O ports are assigned. – 03BCh, 0378h, 0278h. – LPT1, LPT2, LPT3 n Interrupts provided – 07h and 05h

Centronics Connectors (25pin) n On 25 pin connector - IEEE 1284 – 1 - strobe – data bits 0-7 – 10 - Acknowledge – 11 - Busy – 12 - Paper Out – 13 - Select – 14 - Auto LF – 15 - error – 16 - Printer Initialize – 17 - Select inpu – Ground

Parallel Port Signals n Data lines - maintain the bit through the transfer period n Strobe Line – negative pulse - print character – 1 microsecond long n Busy – high = Printer is busy

n Acknowledge Line – normally high - strobes low to signal that printer is ready n Select – high = printer is on line n Paper Empty – high = printer is out of paper

n Fault (error) – high = printer has some problem not identified above n Initialize Printer – low makes printer reset itself n Select Input – low = printer should select itself

n Autofeed – low = automatically send LF after DR

Bi-directional Parallel Ports n Dependent on the configuration of the PPI chip used n can accept 8 bit data on lines n Bus Mastering was added to allow DMA transfer to/from devices connected to parallel ports.

Enhanced Parallel Ports n EPP - a faster port that has a streamlined logical interface and explicit definition of electrical parameters n Standard PP uses three input/output ports for control and moving data -- EPP uses 8 n base + 3 = EPP address port – Signals that the data bits now contain an address

EPP n base + 4 to base +7 = EPP Data buffer – allows host computer to write 32 bit double word to all 4 buffers simultaneously n the 25 pin connector is assigned as a standard parallel port n When Enhanced mode is enabled - only 8 data and 5 signals are used.

EPP signals n Write - pin 1 (low = write is occuring) n Data Strobe - pin 14 (data valid) n Address Strobe - pn 17 (address info is available) n Wait - pin 11 (busy) n Interrupt - pin 10 (high = request interrupt) n the other four ports are used as normal

EPP CABLE n due to the high speed nature of the port, the cable is specifically engineered to reduce noise. n Double-shielded n ~ 2 Mbytes per second – can expect to increase to 8 Mbytes /sec.

Extended Capabilities Ports ECP n adds 2 modes – fast two-way communication mode – data compression (RLE) n Allows for 128 addresses n Additional control registers at 0400h higher than the base registers n 0402h - Extended control register - sets mode

Modes of ECP Table NameAddressModeFunction DataBasePC, PS/2Data Reg ecpAFifoBaseECPEDP FIFO buffer DSRBase+1AllStatus Register DCRBase+2AllControl Register cFifoBase+400EPPEPP FIFO (data) Buffer ecpDFifoBase+400ECPECP FIFO (data) Buffer tFifoBase+400TestTest FIFO cnfgABase+400ConfigConfig. Register A cnfgBBase+401ConfigConfig. Register B ecrBase+402AllExtended control reg.

n backwards compatible n in ECP mode - can write 8, 16, or 32 bits wide. n Allows up to 128 devices on a single port

Speeds n conventional port up to 250,000 bytes per second – with overhead expect 80-90kbps n EPP can produce approx. 800 kbps – can expect up to 8 mbps in the future n ECP incorporates compression – Run Length Encoding – sends byte + multiplier

Assignment