Analyzing Bus Signals in an OpenBus System

Being able to interrogate bus signals between two devices in a system can be very handy from a debugging perspective. In the schematic world, this is done simply by 'tapping off' the signals you want to look at and feeding them to the appropriate instrument, such as a logic analyzer. In the OpenBus System, the required bus signals are made available to the schematic sheet courtesy of the Dual Port Connector component (Figure 1).

Figure 1. Dual Port
Connector component.

The connector carries a master port and a slave port. Simply wire the connector 'in-line' for the particular bus you wish to monitor. In Figure 2, two Dual Port Connector components have been wired into the system at points that will allow observation of how the VGA_TFT Controller and TSK3000A processor race for memory via the Arbiter component.

Figure 2. Example use of Dual Port Connector components in an OpenBus System.

Double-click on a connector to access the Configure (OpenBus Dual Port Connector) dialog (Figure 3). From here, you can specify which specific pins of the interface you wish to have exposed outside of the OpenBus System document.

Figure 3. Configuring the Dual Port
Connector component.

Upon synchronization of the sheet entries and ports for the sheet symbol on the top-level schematic, the bus signals will be made available. Simply wire the required signals to the instrument you are using.

The Dual Port Connector essentially breaks the OpenBus link within which it is placed. Both sides of the link (master and slave) are exposed at the schematic level. In order to keep continuity of the link within the OpenBus System document, you must ensure that the bus is routed back into the system. This is achieved most simply by looping the output signals back to the input signals for the bus (Figure 4).

Figure 4. Example exposure of bus signals for analysis using a Logic Analyzer instrument.