The TX81Z is a rack version of Yamaha DX11
https://en.wikipedia.org/wiki/Yamaha_TX81Z
The TX816 is? a rack unit that could take up to eight TF1 modules. A TF1 module is basically a DX7 condensed down to a single circuit board with almost no front panel controls.
http://www.vintagesynth.com/yamaha/tx816.php
