Tried loading from the serial port and had issues again.

Found the problem is related to the inability of the n_rts line to respond early enough to slow the USB (FPGA to Host) handshake in time. The bufferedUART.vhd code was set to turn off RTS when there are > 8 chars in the receive buffer. The FT230X part doesn't react that fast. Changing the threshold to 4 fixed the problem.

    -- RTS with hysteresis
    -- enable flow if less than 2 characters in buffer
    -- stop flow if greater that 4 chars in buffer (to allow 12 byte overflow)
    -- USB-Serial is fast, but not fast enough to make this larger
    process (clk)
    begin
        if rising_edge(clk) then
            if rxBuffCount<2 then
                n_rts <= '0';
            end if;
            if rxBuffCount>4 then
                n_rts <= '1';
            end if;
        end if;
    end process;

I can now download code again.

Download Code Procedure

• Run puTTY on the Serial port
  • puTTY conflicts with the USB Blaster so it has to be shut down 
  • puTTY should be set to no add LF or CR
  • Assuming the file already has CR LF on each line
  • Can check by puTTY settings, Terminal Implicit LF or CR turned off
• Open the download file in a text editor
  • I like notepad++
  • Make sure the last line in the file to download is
    • S9032000DC
    • If it's not, add the S9 line with a CR at the end of the line
  • Select all, copy to copy-paste buffer (CTRL-C)
• Type LO2 on the VDU keyboard.
• On the Serial port, insert the text with CTRL-V
• After the download, TUTOR will return to the prompt
• Ready to run the program
  • GO addr