Wednesday, January 18, 2017

Building a 68000 Single Board Computer - ACIAs (schematic page 8)

See https://github.com/jefftranter/68000/blob/master/TS2/v2/ts2.pdf

The only I/O ports implemented on the TS2 CPU module are the two 6850 ACIAs shown on schematic page 8. The circuit is almost identical to that found in the ECB module. One port is dedicated to the terminal (IC U29 at address $00010040) and the other (IC U30 at address $00010041) is dedicated to the host computer interface.

The baud rate can be selected by selecting the appropriate output of the baud rate generator U31, but there is little reason to use a rate lower than 9600 bps. The two ACIAs can also be run a different baud rates if desired.

Omitted from my design is the serial port transparent mode feature of the original TS2, also present on the ECB, which connected the terminal interface directly to the host port whenever RTS from U29 was high. This means that the TS2 monitor TRAN and TUTOR TM commands will not operate correctly, but the transparent mode is of little use unless the console is actually a dumb terminal rather than a host computer.

An error should be noted on the schematic on page 896 of the Clements book. The chip selects for the ACIAs should be driven by the signal CS_PERI2* and not CS_PERI1* as shown in the book.

My design also replaces the 1488 and 1489 RS-232 line drivers and receivers with an FTDI connector to support FTDI USB to serial devices rather than true RS-232, which would require the connected computer to have RS-232 serial ports. This also removed the need for +12V and -12C power supplies.

Optionally the board it can be powered by USB using either of the FTDI serial ports - a jumper needs to be connected on the port where power is taken from, in which case an external power supply should not be used.

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