A design tool has been developed in the form of a computer simulation package to aid in the design and performance evaluation of communications controllers with mixed traffic. Such systems are assumed to have the capability of carrying synchronous traffic, asynchronous traffic, or the mixture of both. Multiplexing as well as switching between different types of traffic are among other features provided by these controllers. The hardware/software model is first introduced, and then the computer simulation program is presented along with some results related to the system throughput and delay. A direct application would be in a word processing environment where both inquiry/response as well as file transfer traffic exist.>