October 23, 1997

Signal-swapping layout simplifies debugging

Mike McKean, Motorola Semiconductor, Phoenix, AZ

One pesky challenge that development engineers often encounter is the seemingly simple task of connecting the target system to a dumb or ASCII terminal for debugging or development. As the saying goes, "All things are relative." This adage applies well to communications systems and especially well to transmit and receive pairs: transmit and receive relative to what? Invariably, a mistake occurs in the board-layout connections to and from the connector or driver IC. These mistakes translate into time in the lab for the engineer or technician to swap the transmit and receive pins. The layout in Figure 1 allows you to plan for the problem so you are ready for a quick and easy solution on the fly.

The method works for any two-wire system that may need swapping. In fact, it even protects against possible incorrect cable wiring. The configuration is useful for breadboards or development systems that are more permanent in nature or for systems that multiple individuals use. For every transmit/receive pair, you should implement the 2×4 header (Figure 1). If you need to have the RX signal on Pin A and the TX signal on Pin B, then you should insert 1×2 jumpers as shown. To swap the transmit and receive signals as presented to the ASCII terminal, simply move the 1×2 jumpers to the other vertical position. (DI #2104)

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