It is possible to have multiple buffers in connect to a common node, but care must be taken when designing a system.

Figure 8-3 Connections to Common Node

It is important to try and avoid common node architectures. The multiple nodes sharing a common node can create glitches if the output voltage from a controller target device plus the offset voltage of the buffer are high enough to trip the RTA. Also keep in mind that the V_OS must be crossed in order for a device to begin to regulate the other side.

Consider a system with three buffers connected to a common node and communication between the Controller and Target B that are connected at either end of buffer A and buffer B in series as shown in Figure 8-3. Consider if the V_OL at the input of buffer A is 0.3 V and the V_OL of Target B (when acknowledging) is 0.36 V with the direction changing from Controller and Target B and then from Taarget B to Controller. Before the direction change the user should observe V_IL at the input of buffer A of 0.3 V and its output, the common node, is ~0.36 V. The output of buffer B and buffer C would be ~0.42 V, but Target B is driving 0.4 V, so the voltage at Target B is 0.4 V. The output of buffer C is ~0.52 V. When the controller pull-down turns off, the input of buffer A rises and so does its output, the common node, because it is the only part driving the node. The common node rises to ~0.5 V before the buffer B output turns on, if the pull-up is strong the node may bounce. If the bounce goes above the threshold for the rising edge accelerator ~0.6 V, the accelerators on both buffer A and buffer C will fire, contending with the output of buffer B. The node on the input of buffer A goes high as will the input node of buffer C. After the common node voltage is stable for a while, the rising edge accelerators turn off, and the common node returns to ~0.5 V because the buffer B is still on. The voltage at both the Controller and Target C nodes then fall to ~0.6 V until Target B turned off. This does not cause a failure on the data line as long as the return to 0.5 V on the common node (~0.56 V at the Controller and Target C) occurred before the data setup time. If this were the SCL line, the parts on buffer A and buffer C would see a false clock rather than a stretched clock, which causes a system error.