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STEbus PC-compatible COM and LPT ports

STEbus PC-compatible COM and LPT ports

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This board went through many revisions as the IBM PC market drove the higher integration of PC support chip sets. Originally two 16550 UARTs and an LPT chip, those were replaced by a CHIPS device in an SMD package.
I have the manual and circuit diagram for the board with separate chips for each port (two 8250 style UARTs and a Paradise Systems PPC1 for the parallel port), and photographs of a board that has them all on one chip (CHIPS F82C712 ?).
To recreating this card I am using the Texas 16C552 chip in PLCC68 package, as it is still available.

Eventually PC ISA cards had chips that had COM1, COM2, LPT1, integrated with game ports, floppy disk and hard disk controllers. The floppy drive interface had DMA signals that could not be adapted to the STEBus interface, and were carried to the SCPC88 on a 10-way ribbon cable.

More advanced STEbus boards with 386 and 486 processors had these interfaces integrated into their own chip sets, and did not require a separate board.

Two PC-compatible serial ports and a parallel port are still very useful because commercial evolutionary pressures drove the UARTs to far higher performance to UART chips specific to particular processor families like the 6502 or Z80. The PC UARTs evolved bigger FIFO buffers and automatic handshaking to cope with higher data rates. Many devices used the PC printer port to do things beyond its original purpose, such as scanners, device programmers and hobby projects. Having a printer port would allow such devices to appear again.

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  • 1 × 16C552 Two serial ports and one parallel port

  • Prototyping

    Keith02/23/2022 at 21:56 0 comments

    To avoid a lot of hand-wiring, I bought an 8-bit ISA bus board from eBay. This reduces the effort to interfacing an STEbus slave prototyping card to an 8-bit ISA bus connector.

    There is some TTL and a PAL16L8 doing some address decoding logic.
    The RS232 buffers are 75185 chips. 
    The UART crystal is 3.6864 MHz, and is halved to drive the 16C552. This limits the maximum baud rate to 115200 baud.

    I could solder wires directly to the edge connector, but fitting an ISA bus socket would allow me to experiment with other ISA bus cards without soldering and desoldering. So I bought some sockets.

    Most of the signals are address, data and power lines. The remaining signals are 8088 style, and it should not be hard to create them from the STEbus signals.

  • Choice of chip

    Keith02/23/2022 at 21:03 0 comments

    Many chip manufacturers quickly rushed to make high-integration chips for the IBM PC market. Serial ports, parallel ports, game ports, floppy drive and hard drive interfaces. Any combination of some or all of these functions. When the ISA bus became obsolete, the chip manufacturers left the market just as quick.

    For example, I have an ISA bus multi-I/O card with serial, parallel, floppy and hard drive interfaces provided by a single chip - the Acer M5105. Interesting but not only can I not buy one on eBay, I can't even find the data sheet. No point in designing projects that use unobtainable chips.

    However, chips that provide PC-compatible serial interfaces are still made. They are pretty capable devices, and there had been no market demand to drive the evolution of devices like the 6850, SIO,  and so on. 

    The 16C552 is one of the few chips still made that have an LPT port. Even if you don't have a Centronics printer to talk to, an LPT port can provide a little parallel I/O to play with.

    At the time of writing, the 16C552 is available from Mouser and Digikey.

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