Question

In long-distance digital phone transmissions, the information travels in series (bit by bit) over the telephone network and is clocked at the rate of a clock. When transmitting this information, it can happen that long sequences of ’1’ are sent. Therefore, they can show a DC component on the line (twisted pair, coaxial cable) that may be filtered by the installation (presence of transformer). To avoid such filtering, the natural binary code is transformed into AMI (Alternate Mark Inversion) code. It is a coding technique that uses bipolar pulses to represent logical states. • A logical 0 is represented by no pulse: 0V. • A logic 1 is represented by an alternating polarity pulse (alternately positive and negative): + Vcc or -Vcc. Figure 1: Waveform of a data transmitted following the AMI algorithm. The use of an alternative coding prevents the appearance of a DC component in the conductor. An example of transmited data is shown on the Figure 1. It is proposed to make the next encoder which will transform the binary code to A.M.I. code: The encoder receives as input the data to be transmitted Din, Din H P M AMI Coder Figure 2: Waveform of a data transmitted following the AMI algorithm. and the clock signal H. It outputs two binary signals P (plus) and M (minus) such as: • If Din = 0 ? P = M = 0 • If Din = 1 ? P = 1 and M = 0 or P = 0 and M = 1 alternately. It should be noted that the system must ”remember” the polarity of the last pulse transmitted to function properly. (a) Draw the state diagram for the AMI system. (b) Encode the states and convert your state diagram to a state transition table; derive the final implementation with minimum number of logic gates and D flip-flops. (c) Draw the circuit for the system. 2 It is now desired that the outputs of the encoder are generated directly from the flip-flops output of the state register (i.e. without any combinational logics at the outputs). (a) Propose a solution by clearly specifying the number of scales D flip-flops required. (b) What are the respective advantages and disadvantages of the two solutions studied?

Answer 1

we need 2 flipflops to implement if P and M if outputs are tapped from state register i.e from flipflops(2²⁼4 states.

Here we need to go with moore state diagram..hence more number of states are required , 2 flipflops whereas in mealy type ( 1 flipflop , 2 and gates and 1 xor gate is required)--reduction in number of flipflops..

where as in moore type..(2 flipflops are required, 2 and gates .. and 1 xor gate)

or if only with respect to states...just look at states Q1 and Q0 of flipflops...if (Q1 Q0=01,, P is 1) if (Q1 Q0=11 ,, M is 1)..so in this case there is no need to use any gates (2 and gates shown in circuit diagram below))..here cost is increased w.r.t number of flipflops..i.e. 2 flipflops