Keyboards (was: Re: best dos machine)

The tactile feedback should be quite similar between those 2. Although I must admit I was disapointed when I took a type M apart and found it ws a membrane switch mechansim at heart :-(. I was also disapointed when I fond the HP9830 used keyswtiches. The smaller machines in the range (9810, 9820) used a wonderful electromagnetic system. Under each key were 2 spiral traces on the PCB forming a transformer. 2 keys were conencted in inverse series, so the induced signals in the secodnary 'windings' cancelled out. Pressing a key brought a metal disk onto the associated 'trasnformer', damping it, so the signal from the other one got through. Very reliable [1]. The main problem was it didn't easily hadnle mutluipt keypresses, which is I guess why it wasn't used on the 9830 (whioch had a shift key), but... [1] The electroncis is quite compleicated, but not hard to repair. The comapratoer IC in the sense circuit is hard to find now, though. The contact mechanism in the VT100 keybaord is also used in the VT5x, HP85 (etc), HP9816, some HP terminals, TI99/4A, early TRS-80 Mdoel 1's, etc, etc, etc. A lot fepends on how well it is debounced :-) Odd. I hate the feel of the emchanical HP-HIL keyboards (is that a 46020?) The laster one (46021) which is an odd capacitive membrane thing is perhaps a little better, but not much. -tony