This homebrew dual serial port board for the SS-30 bus provides:
When I upgraded my SwTPc system from the MP-A2 6800 CPU board to the MP-09A 6809 CPU board, I needed a new serial card. The MP-09A's SBUG-E 1.5 monitor didn't support the MP-C serial card with its bit-banged 6820/PIA, only ones that used the 6850/ACIA chip. I modified my motherboard to support extended (20-bit) addresses, which meant the baud rate clock generator on the MP-09A had to be removed, leaving the 8 I/O slots without the 5 clock signals. As a result, a SwTPc MP-S card couldn't be used without providing another source of baud rate clocks.
The newer SwTPc motherboards had an MP-ID card that provided the baud rate clocks. Newer GIMIX motherboards had a slot for a separate baud rate generator board. I don't know how other owners of SwTPc systems provided the baud rate clocks for their serial port cards when they upgraded to the 6809 and modified their older MP-B motherboards to enable extended (20-bit) addresses.
I decided to solve both issues by building my own serial port card with a 6850/ACIA and its own baud rate clock generator. As a bonus, it would have not one, but two serial ports on the one card. All five RS-232 lines are supported and present on the I/O connector: Tx Data, Rx Data, /RTS, /CTS, /DCD. Wiring connects the 12-pin Molex connector with two DB-25 connectors on the rear panel. (Recently I rewired this to use more modern DE-9 connectors.)
However, using this card with a 6800 CPU board and SwTPc's SWTBUG monitor ROM would require modifying the SWTBUG code. Apparently having two 6850/ACIA chips in the same I/O slot confuses SWTBUG's automatic detection of MP-C vs. MP-S cards. But I never used this card with the 6800.
I haven't yet found my schematics for this board.
Pictures of my dual-port serial card.
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