You could try MoSlo, I¡¯ve used it long ago to slow down PCs programming old radios.
you can get the basic ver for free, or delux for donation to a charity for $25.
Also using a good quality USB to serial adapter such as one from Sealevel
79.00, but you can usualy find used ones on eBay. If not sealevel, be sure to use one with a FTDI chip set. Can be found on Amazon, no name for $12 or Star Tech for $33
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Greg
AI?B