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1. CompuTrein Software
2  CompuTrein ISA outputcard
3. CompuTrein Bufferprint
CompuTrein ISA output card
The output card is designed around three 8255A PPI's (Programmable Peripheral Interface, see 8255 datasheet, opens in new browser window if you have Adobe Acrobat Reader version 3 or up installed). Each 8255 has 24 programmable inputs/outputs. The CompuTrein program uses 48 outputs, this means that 16 inputs/outputs are available for later expansion. All 48 outputs are buffered and electrically decoupled by optocouplers.

Address Selection
The address selection of the 8255's (U17..19) is created with a 8-bit comparator (U2) and a de-multiplexer (U4a). A0 and A1 are buffered by U16 and are led to the A0 and A1 inputs of the 8255's to select one of the 4 registers in a 8255 (see 8255 datasheet). The de-multiplexer (U4a) uses A2 and A3 to select one of the 8255's. The outputs of U4a are connected to the Chip Select (CS) inputs of the 8255's. The 8-bit comparator U2 uses the address lines A4 to A9 and the IO signal to activate the de-multiplexer (the IO signal is active as long as IORD or IOWR is active, U4b). The de-multiplexer is only activated if the Address Enable (AEN) signal from the ISA bus is low (address on the bus is available). The comparator output is low when the inputs P0..7 are equal to the inputs Q0..7 (P=Q). If this output is low it activates the de-multiplexer which selects one of the 8255's.
The state of the Q0..2 inputs of the comparator U2 is selectable by means of the dil-switch SW1. In this way the start address of the output card can be selected. If necessary more than one output card can be mounted in one PC as long as they all have a different starting address and the addresses do not conflict any other device in the computer. The selectable addresses are shown in the table below.

SWITCH SW1 8255A U17 8255A U18 8255A U19
ON ON ON 300h 301h 302h 303h 304h 305h 306h 307h 308h 309h 30Ah 30Bh
ON ON OFF 310h 311h 312h 313h 314h 315h 316h 317h 318h 319h 31Ah 31Bh
ON OFF ON 320h 321h 322h 323h 324h 325h 326h 327h 328h 329h 32Ah 32Bh
ON OFF OFF 330h 331h 332h 333h 334h 335h 336h 337h 338h 339h 33Ah 33Bh
OFF ON ON 340h 341h 342h 343h 344h 345h 346h 347h 348h 349h 34Ah 34Bh
OFF ON OFF 350h 351h 352h 353h 354h 355h 356h 357h 358h 359h 35Ah 35Bh
OFF OFF ON 360h 361h 362h 363h 364h 365h 366h 367h 368h 69h 36Ah 36Bh
OFF OFF OFF 370h 371h 372h 373h 374h 375h 376h 377h 378h 379h 37Ah 37Bh

The data lines of the ISA bus are buffered with bi-directional ports which are activated by the address selection. The direction of the ports is depending on the IORD signal from the ISA bus. The data lines are connected to the data bus of the all three 8255's.

The RST signal of the ISA bus is buffered in U16 and connected to the RESET input of the 8255's. If the reset goes high the 8255's are set to their initial state.

The outputs of the 8255's U17 and U18 are buffered by U1, U5..7, U14 and U15. The selection inputs G1 and G2 are connected to ground. The outputs of the buffers are connected through a resistor to the optocouplers U8..13 and U20..25 (each block consists out of 4 optocouplers). The transistors of the optocouplers are used as an open-collector output which are available on connector J1 at the rear of the computer. The inputs/outputs of the 8255 U19 are available on connector J3 (inside computer) for later expansion.


Click on the schematic to view the full size

Part list Output card:
  • R1..48: 1k
  • R49..51: 4k7
  • U1,5,6,7,14,15,16: 74HC541
  • U2: 74HC688
  • U3: 74HC245
  • U4: 74HC139
  • U8..13,20..25: 4x P620
  • U17..19: 8255A
  • J1: 50p flat cable connector
  • J2: ISA bus connector
  • J3: connector
  • SW1: 3p dil-switch
  • C: 12x 100n*
*Gray parts are not specified in the schematic

If you have any questions, suggestions or remarks on this description or on the schematic please send me an e-mail and I will try to respond.

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