Heidenhain DRO - 7pin to 9 pin?

I've got a mill with an old Heidenhain DRO, LS500 scales and the
simple low, flat display like a cassette deck. The display has failed
on one axis.I picked up a slightly later counter, type VRZ 731B
thinking it might do the job, but the sockets for the scales are
9-pin
as against 7-pin on the existing scales.
Anyone know if the two are fundamentally incompatible, or whether
they
could be made to work together with a bit of rewiring?

Thanks

Tim