Network termination

To prevent reflection of the signals at the ends of the network the lines must be correctly terminated. In general the termination required will vary with network geometry, speed and length. On slower or shorter networks the termination is not so critical . When installing long or fast networks it is recommended that the signals on the line are viewed with a differential input oscilloscope while the terminations are being adjusted. The termination is correct when the waveforms on the lines are sharp square waves with no ringing or rounding. The waveforms are most easily observed initially on the clock line and this should be adjusted first. Twisted pair cable as recommended for the network normally has a characteristic impedance in the order of 100 ohms. The line terminators should therefore present 100 ohms across the line. There is a further requirement that the data line when undriven should rest in a state which will be interpreted as a logic I by the receivers on the network and so the termination circuit also applies some bias on to the data wire pair.

The clock line will usually be terminated correctly by linking across positions for R3 and R4 and inserting a 100 ohm resistor in the space marked D 2. The data line will normally be terminated by inserting these components in the marked positions:-

	R 1, R 2	56 ohm  resistors
	D 1		1N4148  diode
	C1, C 2		10 uf capacitors
	R 5		100 ohm resistor
	D 3		100 ohm resistor
	D 4		wire link