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Circuit Description

This applet demonstrates the PIO 8255 or parallel input/output adapter. The PIO allows connecting a microcomputer system to a variety of external devices - with very few external components.

The simulation model shown here is based on the Intel 8255 chip, which was originally developed for systems based on the 8080/8085 series 8-bit microprocessors, but can be attached to most microprocessor system buses. For example, the 8255 chip was used for the parallel port (or printer port) in the original IBM PC. In current generation PCs, the logic that drives the parallel port is integrated into the mainboard chipset or a single so-called super-IO chip to save costs, but the behaviour is still fully compatible with the original 8255 chip.

This applet is used to interactively explore the functions of the 8255. Once the simulation has been loaded, you will have to generate a reset pulse to initialize the chip. Afterwards, you will have to load the control word into the control register at address three to configure the input/output mode and port directions. See the table below for a basic function description. For the full details, please download and read the original datasheet.

For a first introduction, please visit the following 8255 animation applet, which demonstrates a fixed sequence including device initialization and several I/O operations with text-to-speech explanations. For a typical microprocessor system using the 8255, click here.

To control the applet, either click the input switches, or type the following bindkeys: 'a' and 'b' for the address inputs A0 and A1, 't' for reset, 'c' for chip-select, 'r' for read, and 'w' for write. Type '1', '2', '3', and '5' to control the 'X' input switches connected to the corresponding lines of port B. To restore the 'X' input switches connected to port B to their tristate (Z) state, hold down the shift-key and click them (shift+click). Remember to use click/shift+click/control+click combinations, or use the property-sheet (popup-edit) to control the DATA bus input switch. The switch should be set to the tristate state during read operations (use control+click).

8255 PIO description

  reset  nCS  nRD  nWR  A1  A0  data  |  behaviour
 -------------------------------------+-----------------------------------
      1    *    *    *   *   *     *  |  device reset (mode 0, all inputs)
      0    1    *    *   *   *     *  |  device passive
      0    0    1    1   *   *     *  |  device selected
      0    0    0    1   A1 A0  read  |  read selected register or port
      0    0    1    0   A1 A0  write |  write selected register or port

      u    u    u    u   u   u     *  |  any undefined U,X,Z value will
                                      |  invalidate the simulation model

The chip contains four registers which are selected by two address inputs A1 and A0 during the read and write operations:

  A1 A0    
   0  0    port A data register
   0  1    port B data register
   1  0    port C data register
   1  1    control register

The 8-bit data written to the control register configures the 8255 and selects the operation mode, based on the following mapping:

   bit
    D7     1=mode selection 0=single-bit set/reset
    D6D5   groupA mode:  00=mode0 01=mode1 1*=mode2
    D4     portA:        0=output 1=input 
    D3     portC-upper:  0=output 1=input
    D2     groupB:       0=mode0  1=mode1
    D1     portB:        0=output 1=input
    D0     portC-lower:  0=output 1=input

The three modes are:

  Mode 0: for basic input and output operations,
  Mode 1: for unidirectional data-transfers with automatic handshake,
  Mode 2: for bidirectional data-transfers with handshake.

Please note that our simulation model only supports mode 0 at the moment. Any attempt to select mode 1 or mode 2 is ignored. Of course, you are welcome to write the additional logic yourself... Please let us know when you do so :-)

For example, writing the data word 0x80 (10000000) into the control register selects mode 0 for all ports and configures all ports as outputs. Afterwards, data written to one of the three data registers will appear immediately on the corresponding pins. On the other hand, writing the data word 0x9b (10011011) into the control register selects mode 0 and configures all ports as inputs. Subsequent read operations of the data registers will return the current input values at the corresponding pins. As a third example, the control word 0x89 (10001001) selects ports A and B as 8-bit outputs, the upper half of port C as a 4-bit output, and the lower half of port C as a 4-bit input. Subsequent writes to the port C data register will now drive the upper four bits of port C, while writing the lower four bits (configured as inputs) is ignored. Similarly, reading the port C data register will return the current four input values of the lower half of port C, merged with the upper four bits from the last port C write operation.