O ct 1 99 8 Tau - function for discrete sine - Gordon equation and quantum R - matrix

We prove that the τ-function of the integrable discrete sine-Gordon model apart from the " standard " bilinar identities obeys a number of " non-standard " ones. They can be combined into a bivector 3-dimensional difference equation which is shown to contain Hirota's difference analogue of the sine-Gordon equation and both auxiliary linear problems for it. We observe that this equation is most naturally written in terms of the quantum R-matrix for the XXZ spin chain and looks then like a relation of the " vertex-face correspondence " type.