The present invention relates generally to communications systems and methods. More particularly, the present invention relates to a system and method for providing bidirectional data communication between a master device and one or more isolated slave devices.
Serial communication buses are widely known for transferring data from one device to another or to multiple devices. Such a communication bus can be used for example to transmit configuration data from a system controller to a power supply or to receive monitoring data from the power supply itself. The communication bus can also be used to implement a communication channel within the power supply itself, for example to configure or monitor different outputs of the power supply.
In order to reduce the number of physical signal lines, a wired-AND bus configuration 100 in conjunction with open-drain drivers and a passive pull-up device can be used such as shown for example in FIG. 1. When the master and slave devices 102, 104 are not communicating the signal lines are pulled high and free for communication. Each device 102, 104 can subsequently pull the signal line to a low voltage level to initiate communications in accordance with a particular sequence in which the signal line is pulled low. Such configurations are used as known in the art for example in I2C buses or in the SMBus. These configurations are advantageous in that with an appropriate protocol data can be transmitted in both directions on the same signal line, without requiring explicit handshaking. Further, wired-AND configurations are not limited to one transmitter and one receiver, as several receivers may be connected to the same signal line.
When such a bus is used within a power supply to transfer data from an on-board power supply controller and several output modules such as for example in a modular AC-DC power supply, then it becomes necessary to provide a galvanic isolation between the transmitter and each of the receivers. Solutions to provide a galvanic isolation for a wired-AND communication bus have been proposed and are well known in the art. An example of such a solution using cross-coupled isolators IC120, IC121 associated with each of master and slave devices 112, 114 and coupled along a communications bus 116 is shown in the communications bus configuration 110 of FIG. 2.
The difficulty in circuits such as that shown in FIG. 2 is to make sure that the cross-coupled isolators 120 which are used to transfer the communication data in both directions do not form a latch. This typically requires adding some amount of circuitry in order to be able to distinguish in which direction the data is sent so that only one of the opto-isolators 120 is used at the time, thus avoiding a condition where data such as for example an output low drive signal transferred in one direction gets immediately sent back and blocks the entire communication channel by holding the input low even after the output low drive signal is no longer present. A signal line “latched” in such a manner may require complex circuitry to break the loop, or may cause further potential problems such as oscillation where multiple lines are held low and then released at the same time.
Referring now to a circuit example 200 such as shown in FIG. 3, in cases where more than two devices are connected to the same bus, the proposed isolator configurations are becoming onerous in their complexity. A pair of isolators 220 are provided for each secondary device 216, and an encoder/decoder pair 218, 222 are further provided on either end of each isolator pair 220 to facilitate the bidirectional communications.
However, in many cases the first device 212 will be configured as a master device 212 talking to, and getting response from, the other devices 214. The first device 212 could be for example a power supply controller 212, whereas the secondary devices 214 could be PWM controllers 214 for a specific isolated power supply output of the power supply system. In this case there is also usually no need to communicate from one output device 214 to another output device 214. There will only be communications along the bus 216 directly between the power supply controller 212 and the individual PWM controllers 214. It would be desirable in these cases to simplify the isolation scheme and reduce circuit duplication.