Data Acquisition

transceiver in the MAU and this is fed with power from the NIC via the AUI cable. Sincethe transceiver is remote from the NIC, the node needs to be aware that the terminationcan detect collisions if they occur. This confirmation is performed by a Signal QualityError (SQE), or heartbeat test function, in the MAU. The SQE signal is sent from theMAU to the node on detecting a collision on the bus. However, on completion of everyframe transmission by the MAU, the SQE signal is asserted to ensure that the circuitryremains active, and that collisions can be detected. You should be aware that not allcomponents support SQE test and mixing those that do with those that don’t can causeproblems. Specifically, if a NIC was to receive a SQE signal after a frame had been sent,and it was not expecting it, the NIC could think it was seeing a collision. In turn, as youwill see later in the manual, the NIC will then transmit a jam signal.The other type of coaxial cable Ethernet networks is 10Base2 and often referred to as‘Thinnet’ or sometimes ‘thinwire Ethernet’. It uses type RG-58 A/U or C/U with a50 Ω characteristic impedance and of 5 mm diameter. The cable is normally connected tothe NICs in the nodes by means of a BNC T-piece connector, and represents a daisy chainapproach to cabling. Connectivity requirements include:• Termination at each end with a 50 ohm terminator• The maximum length of a cable segment is 185 meters and NOT 200 meters!• No more than 30 transceivers can be connected to any one segment• There must be a minimum spacing of 0.5 meters between nodes• It may not be used as a link segment between two ‘Thicknet’ segments• The minimum bend radius is 5 cmThe physical layout of a 10Base2 Ethernet segment is shown in Figure 9.4.Figure 9.410Base2 Ethernet segment