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Does your amp accept 4-line "BCD" band data or does it insist on using a serial data stream? I've always used the band data from the K3 ACC socket, cascading true-15 pin Y-connectors as necessary to provide various signals to different devices. Works fine and doesn't depend on serial data stream timing.
If the amp insists on RS232 for band data, does it listen only (like a SteppIR controller) or does it insist on "talking" on the RS232 line? If it's listen-only, then you can cascade a 9-pin Y-connector.
If the amp insists on a bi-directional RS232 link, consider the various products that simulate multiple serial ports on a single computer input. I haven't used them and am just vaguely aware that they exist, but that might be a place to look if no other alternatives work.
GL & 73,
Rick Tavan N6XI