Programable I/O expanding decoder using 8255 PPI part-2

HARDWARE

            The decoder card used ISA slot (8-bit expansion card), so the card will be reside in the PC. We used only the lower slot (62-pins). To access the line, I used a standard connector DB-25. I used a female type, so I can utilize the standard parallel cable usually selling in the market. But if you decide to used this female type, careful about the same connector between it with LPT parallel port connector. If you can not differentiate it or if the PC used by many people, better used a male type. This can prefent you or someone else from plugged at wrong connector. But the bad side is you must make your own cable. Fig-2. shows the PCB layout. The design is not quiet good, but it works, and also easy to drawing it.

                                                Fig-2. 8255-PPI I/O decoder PCB layout.

Note that if you change the connector to DB-25 male type, all of the wiring layout connected to DB-25 must be changed and drawn again. Because the female and male type count direction are opposite. I used a jumper strip (header) for selecting the IRQn lines, so the card can be selected which number is still available (IRQ-3 to IRQ-7). to select the I/O address, just simple set up the 8-bit dip switch to the selected unused I/O address (refer to I/O map for details). Each dip switch from MSB to LSB (Q7 ~ Q0) related to address lines A9 ~ A2, while A1 and A0 directly selected by the PPI-8255 itself. To select an I/O address, eg.: I/O address 0220 Hex, the dip switch set directly from MSB to LSB (bottom to top at the picture) are : off-on-on-on-off-on-on-on. Just remember that this switch are opposite to the setting bits.

PART LISTS

            The components for building this 8255-PPI I/O decoder are :

   1. Resistors :
      R1 ~ R14 = 4k7 ........................5 pcs

   2. Capasitors :
      C1 = 100 uF/16V (for power input) .....1 pcs

      C2~C4 = 100 nf (ceramic) .............  3 pcs

   3. Semiconductors :
      IC1 = 74LS682 (8-bit comparator) ...... 1 pcs
      IC2 = 74LS32 (Quad OR gates) ...........1 pcs
      IC3 = 74LS245 (Octal tri-state buffer) . 1 pcs
      IC4 = 74LS125 (Quad tri-state buffer) ..  1 pcs

   4. Others :
      Optional IC socket for 20-pins ..... 2 pcs
      Optional IC socket for 16-pins ..... 1 pcs
      Optional IC socket for 14-pins ..... 2 pcs
      Pin header ..........................10 pcs
      Jumper header ....................... 1 pcs
      DB-25 socket female type ............. 1 pcs
      2-layers PCB about 75mm x 105mm size . 1 pcs
      Plate for pcb mounting ................ 1 pcs

 

PROTOTYPE

            Fig-3. shows my prototype. The prototype made by hand. Not too bad. You see! Don't forget about through hole. Hand made PCB don't have a through hole copper. you must make it by connection. I use a tiny wire (usually I took it from a small transformer that already damage). Before you put the components, soldering the wire at the top layer first, and then the bottom layer can be soldered both. Hope you have a nice work!!!

                                                   Fig-3. My protoype board, front and rear side view.

EXPANDER CARD MODIFICATION

            In the previous design lpt expander card, PPI-8255 pin directly connected to LPT port line signal. We must make so the proper connection related to the proper pins. For this purpose some changes must be made to the original design. Pin connection for address selection (A0-A1) and read/write signal (-WR/-RD) are not changed. But for reset and chip select (-CS) the connection pin changed by a double selection switch. This pin connect to the other unused pin connector. I used 'select' pin and 'paper end' pin for reset and chip select respectively. The complete changed new schematic diagram are shown at fig-4. Beside that a push button (push on) switch was added. This button used for resetting the card if the card not reset by the PC warm boot, or if the card not connected when doing re-boot-ing, or the time not happen to be doing re-boot-ting. Just simple push the button once. Remember that, if the button was pushed after the initializing card has done, all accessing to the ports would be inhibited. The rest is the same as before.


click the images for enlarging 

                      Fig-4. LPT expander card modification circuit diagram.

 Fig-5. shows the PCB layout for expander card modification. The elippses show the two jumper wire that must be opened and another track should be added. Opened point goto selector switch (sw1) and push button switch (sw2). Another is IRQn which optional using, if the PPI-8255 configured for mode-1 or mode-2. This point connect to the proper pin of port-C. 

                                        Fig-5. LPT expander card modification PCB layout.

Fig-6. Shows the setting card on the IBM PC/AT compatible (80486DX2-66), and fig-7. Shows the application program to run the 36-bits led display driver.

                                          Fig-6. Setting card on IBM PC/AT compatible, ISA slot.

                                      Fig-7. Application program running 36-bits led display driver.