There are different variants of RBMs, and the differences lie in the types of visible and hidden units we choose as well as in the implementation of the energy function \( E(\boldsymbol{x},\boldsymbol{h},\boldsymbol{\Theta}) \). The connection between the nodes in the two layers is given by the weights \( w_{ij} \).
RBMs were first developed using binary units in both the visible and hidden layer. The corresponding energy function is defined as follows:
$$ \begin{align*} E(\boldsymbol{x}, \boldsymbol{h},\boldsymbol{\Theta}) = - \sum_i^M x_i a_i- \sum_j^N b_j h_j - \sum_{i,j}^{M,N} x_i w_{ij} h_j, \end{align*} $$where the binary values taken on by the nodes are most commonly 0 and 1.