Aug 30, 2021

S100 FPGA and 16550 UART

It has been a while since I’ve written on this blog. There has been quite a lot that has happened since my last update. One of the most recurring theme in this endavor on the retro computing has been constantly repairing old, unreliable circuits. Since now the focus has been slowly shifting to the software side, for the interest of providing a reliable platform for software debugging, I decided to deisng a S100 add-in card that features an FPGA.

Design Goals

Some of the main goal of this FPGA add-in card is to be able to provide a reliable way of emulating any hardware, let it be existing retro hardware or anything we are about to design. It turns out having an FPGA at your disposal really aids in accelerating the design and revision process. Here are some of the requirement we set when designing this AIC (add-in card).

  • Have access to the entire S100 bus and it’s signals.
  • Able to respond to IO reuqests as well as Memory requests.
  • Ability to issue interrupt to the CPU.
  • Efficient power regulation (aka not 7805s)

Implementation

FPGA Card

In the above picture, you can see the first implementation of this design. As most electronic designs are, this first revision is flawed in a few ways (as indicated by the patch wires haha). On the left side of the board, it is a 16550 UART and supporting circutry for it to work in a S100 computer. 16550 became the standard UART chip that can even be commonly found in modern computers and various micro controllers.

On the bottom left corner of the board, you can see the voltage regulation section of the board. Where there are a few sets of power regulation system that derive a regulated +5V and +3.3V for various component on the board from the unregulated +8V rail on the S100 bus.

On the right side of the board, you can see the main component, the FPGA daughter board. Due to the flexibility nature of FPGA, even if in the future this specific FPGA is not availiable anymore, you can very easily design an adapter / redesign the board without much effort to adapt it for another FPGA.

Ending

I think it would be very interesting to see where this addition to the IMSAI can take us. Now having the ability to quickly prototype and potentially debug other hardware on the bus, this FPGA should give us some big help in the future.

Stuff might be cool to implement with this card

  1. Banked memory support
  2. S100 Bus recording / dumping tool. Like a logic analyzer.
  3. Co-Processor (idk something like a tiny RISCV core could be fun)
  4. DMA Engine (I think this is possible with the current board design, but should verify).
  5. Graphics Engine / VGA.