Heartbeat <> SQE

The first Ethernet implementations used external transceivers on 'yellow cable' (today's adapters have an on-board transceiver). External transceivers (as well as AUI drop cables) have special signalling lines. Ethernet II and IEEE 802.3 make different use of these signalling lines:

Every send and transmit operation is acknowledged by the transceiver. If the controller doesn't receive this 'heartbeat' this will be seen as an error condition. This was necessary because the hardware components were instable.

SQE: Signal Quality Error

Later on, this kind of hardware check wasn't necessary any longer. IEEE 802.3 Ethernet transceivers (MAUs = media access units) only send control signals if signal quality errors occur like collision or lost carrier (caused by too many collisions or by short cable). IEEE transceivers always perform SQE tests.

Most pocket transceivers have a switch for heartbeat or SQE test. If the transceiver is set to heartbeat, an IEEE controller won't be able to operate properly.