Transmission Lines For Ttl Circuits

Although cable connections between TTL circuits are nor- mally not as critical as those for, say, RF applications, it is still worth while to reflect on this subject because strange things often happen when a TTL trans- mission line is not correctly ter- minated. In particular, this discussion is about terminating coaxial cable and flat ribbon cable. The latter is frequently used for driving Centronics compatible inputs. A commonly used coaxial cable is RG59B/U, which has a characteristic impedance of 75 Q and a propagation delay of 5 ns/m. With signal rise and fall times of 4 ns, the cable may be considered electrically long if it exceeds 40 cm. One of the most common terminations used when driving a long coaxial cable with an LSTTL gate is shown in Fig. 1. This set-up is unsuitable for a HCT bus driver, since the termination provides a poor impedance match, and requires a current sinking capability of 20 mA. An improved termination cir- cuit is shown in Fig. 2: this ensures reli...