So, I changed all the capacitors and I can't get things back to the way they were. I can send data from the Dx to the Mac (using either serial port), but I can't get data from the Mac to the DX..

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More info please. Can you send any data through your serial ports? Does regular MIDI data drive the keyboard? Can you print? Can you access a modem? If software you've always used worked before, then it should still work unless the hardware is compromised. New software won't change that. It sounds as if you missed something on the logicboard, or did not replace a particular cap correctly.