MATLAB_Production machine failure(MTBF & MTTR)

I am modelling a DISCRETE EVENT SIMULATION (DEV/DEVS) problem. The problem is concerned with FMS(flexible manufacturing system). It involves six unreliable(likely to fail) machines which can fail during operation. 1) I want to know how to model unreliable machines in terms of MTBF & MTTR using exponential distribution. 2) Also, I want to study the effect of varying MTBF and MTTR values on the problem.

e.g. Distribution used: Exponential;MTBF:400,600.....;MTTR:25,50.....