Signalling between independently powered electrical circuit cards

A plurality of circuit cards include at least a first card having a first internal power rail, an earth connection, respective operating circuits powered from the first power rail, external terminals coupled to its operating circuit, and a first external connection. A second of the plural cards has a second internal power rail, an earth connection, respective operating circuits powered from the second power rail, external terminals coupled to its operating circuit, and a second external connector. The first card includes a bipolar transistor with an emitter-base junction forwardly biased in response to normal voltage between the first power rail and the earth connection and a collector connected to the first external connector. The second card includes a resistive connection between its second internal power rail and the second external connector and circuits responsive to changes in voltage level at the second connector.