With the help of a minimal quantity of hardware and a simple piece of software it is possible to tap into the data stream between two serial interfaces and observe it on the computer monitor.The name ‘Data Spy’ immediately conjures up images of clandestine practices, in general however, the circuit described here will not be used for such purposes. There are many legitimate reasons for analysing the data stream between a PC and a peripheral, or a second PC. It could be part of a faultfinding procedure, for example, or as a continuous data monitor, or as an educational exercise to unravel the details of certain protocols.The implementation of the circuit described here is a kind of in-line plug that is simply inserted in series with the connection between the RS232 interfaces of the PC and the peripheral under consideration. A third sub-D connector functions as the connection for the additional PC, which is used to examine the data stream.