Accurate Logic Simulation Models for TTL Totempole and MOS Gates and Tristate Devices

A component finding wide usage in the bus-oriented architecture of today's computer systems is the tristate driver. A typical arrangement is shown in Fig. 1. In this arrangement there are several drivers. Only one driver can be enabled at a time and "talk" to the bus. The receivers capture the information on the bus. In transistor-transistor logic (TTL) technology, tristate devices allow bus wiring, previously obtained only with conventional open collector output TTL. They also allow the use of active pull up to charge the large capacitances associated with the bus. This feature, not available with conventional bus wiring technique, speeds up the operation of the bus. In MOS technology, similar effects can be achieved with lower power requirements. Here, we shall consider T T L totempole, CMOS, PMOS, and N M O S bistate and tristate devices and show the similarities and differences. In tristate technology, several malfunctions due to the presence of faults or to a wrong utilization of the bus may occur. These malfunc1271