In normal mode the calculator worked very, very slowly. For example, the calculation of the sine of 3 required whole 3.5 seconds. It was possible to put the calculator in “accelerated mode” by pressing the “+” key. Then the same calculation took “only” 0.5 seconds, but you could see the batteries literally “decay” in front of your eyes, and very soon you had to replace them.
The MK-85 is much more tuned for end users than the MK-90.
The MK-90 feels like an unfinished prototype (both in hardware) with a demo version of software onboard (just an old PDP-11 BASIC from 1970s with a few addition). The MK-85 closely imitates the Casio FX-700P calculator behaviour (both external appereance and the software functionality) so it is a much better device for everyday use.
Having no first hand knowledege of either of these computer, I’ve to admit that I do share some faszination for the MK-90 (more as a concept than the real machine, which I don’t know). The MK-85, in theory, seemed always a bit ironic to me, using a PDP-11 to emulate a Casio pocket computer.
That said, it seems fair and reasonable that the Casio approach was probably the better fit for the form factor.
For the MK-85 they simply picked the CPU which was easily available at acceptable prices (it the USSR).
The MK-85 is way faster than the Casio (with some work you can enter machine code here - see Piotr Piatek’s pages, for example) . The problem is in the BASIC implementation. Still, the BASIC in the MK-90 is even worse…
As I do unterstand it (I don’t rember whare I did pick this up) the BASIC of the MK-90 does BCD math only. Is this correct? Using a backslash for the statement separator (as in DEC BASIC) might be another one. – Are there any other issues of note?
Another issue is that the MK-90 BASIC requires full syntax (like: 10 LET A = 3 ; you cannot omit the LET) which is annoying because the MK-90 keyboard is far from comfortable. I also didn’t find any BASIC command how to save data to the “disk” (this is stupid as there are two 10 kB storage modulles) and even here is nothing like POKE command.