Rutex do one, dont know of anyone who has tried one though. CNC Teknix were reportedly going to be making some but not seen any info on their website. The Tek10 servos drives top board supposedly can do it and I have actually semi tried it but abandoned my trials when my AB drives came along. When I say I semi tried what I mean was I had an encoder hooked up to the encoder inputs of the top board from a Tek10 and measured the voltage as I rotated the encoder one way or the other and it seemed to act as it should but how good it would have been connected to a drive I cant say. I have a couple of the top boards here and I may use them in the future but if you pay for the shipping to the USA I could loan one to you to see if it will work, PM me if you want to try.
Hood