Topology refers to the alignment order of the wiring and the wiring structure of a network. For multi- network load , according to the actual situation, select the appropriate cabling topologies and take the right " to " Termination is very important. Under normal circumstances , PCB traces can choose several topology as shown.
Several typical topology
( A ) point
Fig 2 (a) shows the point topology is relatively simple , as long as the driver or receiver for proper impedance ( normally suffice to use one , some of the circuit will occur require simultaneous use of two match ) , you can get a better signal integrity.
( 2 ) Daisy Chain
When the entire length of the trace network delay is less than the signal rise or fall time , can be used as shown in Figure (b) in a daisy -chain topology , the wiring from the drive end are sequentially reach the receiving end, the actual design should make a daisy chain wiring branch lengths as short as possible .
Advantages daisy chain alignment are:
· Wiring space occupied by a single resistor and can be used to match the smaller end ;
· In the control traces harmonic interference , the better results.
Disadvantages daisy chain alignment are:
· Cloth pass rate, 100% of the cloth is not easy to pass ;
· The type of signal receiver, the received signals are not synchronized.
( 3 ) Star
When a drive signal to drive a plurality of signal receivers , and requires a plurality of signal receivers simultaneously receive signals to be used as shown in Figure (c) a star topology requires that each branch and the load receiving end of the wiring length as consistent , each branch generally requires termination resistors and terminal resistor should match the characteristic impedance of the connection . Thus even in the case of very fast edge rates can still get good performance.
Star topology can effectively avoid the problem of clock signals are not synchronized , but in the high -density PCB layout manually is very difficult to complete a star can be automatic routing wiring .
( 4 ) remote branch
As shown the distal branches of (d), which is similar with a star , but the branch is close to the receiving end . In this topology , but also limit the length of the distal branch , the branch on the transmission delay is less than the signal rise or fall time .
( 5 ) Periodic load
Cyclic load topology shown in Figure (c) , the required length of each branch is sufficiently small , the delay of the transmission branch of the signal is less than the rise or fall time . This backbone structure and all the branches of the transmission line segments can be seen in combination with a new section of transmission line characteristic impedance of the transmission line than the original main characteristic impedance is small , the transmission rate is lower than the original , and therefore impedance during attention.
In the actual PCB design process, the key signal , signal integrity analysis should be determined by the use of which topology.
