Communication System

1. A communication system using a pulse width modulation code, in which a width of a low level of logic 1 is narrower than a width of a low level of logic 0, as a transmission code and comprising a plurality of nodes each including a driver circuit that includes a pull-up circuit connected between a power source and a transmission line and a switching portion connecting and disconnecting the transmission line and a ground line, one of the nodes being a master node, the others of the nodes being slave nodes, the driver circuits being driven so that the master node constantly outputs the transmission code of logic 1 and the slave node outputting the transmission code of logic 0 extends the width of the low level of the transmission code of logic 1 on the transmission line, wherein the master node further includes a current limiting section that limits an electric current flowing to the transmission line via the pull-up circuit based on at least a signal level of the transmission line, and the current limiting section includes a plurality of resistors, and a switching circuit that switches a connection state of one of the resistors, and increases a resistance value of the pull-up circuit when the signal level of the transmission line is at the low level, and the current limiting section further includes a detection circuit that detects a voltage of the transmission line, and the current limiting section increases the resistance value of the pull-up circuit when the detection circuit detects that the signal level of the transmission line is at the low level.

2. The communication system according to claim 1 , wherein the current limiting section increases the resistance value of the pull-up circuit when the signal level of the transmission line is at the low level and the driver circuit is driven so that an output signal of the driver circuit becomes the high level.

3. The communication system according to claim 2 , wherein the master node further includes a waveform shaping section that restricts a change speed of the signal level when the output signal of the driver circuit changes from the low level to the high level.

4. The communication system according to claim 1 , wherein the current limiting section controls the resistance value of the pull-up circuit among three or more stages.

5. The communication system using a pulse width modulation code, in which a width of a low level of logic 1 is narrower than a width of a low level of logic 0, as a transmission code and comprising a plurality of nodes each including a driver circuit that includes a pull-up circuit connected between a power source and a transmission line and a switching portion connecting and disconnecting the transmission line and a ground line, one of the nodes being a master node, the others of the nodes being slave nodes, the driver circuits being driven so that the master node constantly outputs the transmission code of logic 1 and the slave node outputting the transmission code of logic 0 extends the width of the low level of the transmission code of logic 1 on the transmission line, wherein the master node further includes a current limiting section that limits an electric current flowing to the transmission line via the pull-up circuit based on at least a signal level of the transmission line, and each of the slave nodes further includes a waveform shaping section that restricts a change speed of a signal level when an output signal of the driver circuit changes from the low level to the high level.

6. The communication system according to claim 5 , wherein the current limiting section increases a resistance value of the pull-up circuit when the signal level of the transmission line is at the low level and the driver circuit is driven so that an output signal of the driver circuit becomes the high level.

7. The communication system according to claim 6 , wherein the master node further includes a waveform shaping section that restricts a change speed of the signal level when the output signal of the driver circuit changes from the low level to the high level.

8. The communication system according to claim 5 , wherein the current limiting section controls a resistance value of the pull-up circuit among three or more stages.

9. A communication system using a pulse width modulation code, in which a width of a low level of logic 1 is narrower than a width of a low level of logic 0, as a transmission code and comprising a plurality of nodes each including a driver circuit that includes a pull-up circuit connected between a power source and a transmission line and a switching portion connecting and disconnecting the transmission line and a ground line, one of the nodes being a master node, the others of the nodes being slave nodes, the driver circuits being driven so that the master node constantly outputs the transmission code of logic 1 and the slave node outputting the transmission code of logic 0 extends the width of the low level of the transmission code of logic 1 on the transmission line, wherein the master node further includes a current limiting section that limits an electric current flowing to the transmission line via the pull-up circuit based on at least a signal level of the transmission line, and the current limiting section uses a threshold value having hysteresis for determining the signal level of the transmission line, and the threshold value used for determining a change from the high level to the low level and the threshold value used for determining a change from the low level to the high level are set to be different values.

10. The communication system according to claim 9 , wherein the current limiting section increases a resistance value of the pull-up circuit when the signal level of the transmission line is at the low level and the driver circuit is driven so that an output signal of the driver circuit becomes the high level.

11. The communication system according to claim 10 , wherein the master node further includes a waveform shaping section that restricts a change speed of the signal level when the output signal of the driver circuit changes from the low level to the high level.

12. The communication system according to claim 9 , wherein the current limiting section controls a resistance value of the pull-up circuit among three or more stages.

13. A communication system using a pulse width modulation code, in which a width of a low level of logic 1 is narrower than a width of a low level of logic 0, as a transmission code and comprising a plurality of nodes each including a driver circuit that includes a pull-up circuit connected between a power source and a transmission line and a switching portion connecting and disconnecting the transmission line and a ground line, one of the nodes being a master node, the others of the nodes being slave nodes, the driver circuits being driven so that the master node constantly outputs the transmission code of logic 1 and the slave node outputting the transmission code of logic 0 extends the width of the low level of the transmission code of logic 1 on the transmission line, wherein the master node further includes a current limiting section that limits an electric current flowing to the transmission line via the pull-up circuit based on at least a signal level of the transmission line, and each of the slave nodes includes a level shifting section that sets the signal level of the transmission line when the switching portion included in the driver circuit in each of the slave nodes is on to be higher than the signal level of the transmission line when only the switching portion included in the driver circuit in the master node is on.

14. The communication system according to claim 13 , wherein the current limiting section increases a resistance value of the pull-up circuit when the signal level of the transmission line is at the low level and the driver circuit is driven so that an output signal of the driver circuit becomes the high level.

15. The communication system according to claim 14 , wherein the master node further includes a waveform shaping section that restricts a change speed of the signal level when the output signal of the driver circuit changes from the low level to the high level.

16. The communication system according to claim 13 , wherein the current limiting section controls a resistance value of the pull-up circuit among three or more stages.