Analog Switch Multiplexes SFP Modules

要約

To avoid multiple I2C bus masters in a telecom or data-communications system with SFP modules, add an analog multiplexer (MAX4572) that directs the data line to the appropriate SFP module.

A similar version of this article appeared in the August 2, 2007 issue of Electronic Design magazine.

Small-form-factor pluggable modules (SFP modules) are in common use for the physical-layer interface in telecom and data-communications equipment. Also common in these systems is an I2C bus for the MDIO interface (Management Data Input/Output).

Sadly, when several SFP modules are used they all have the same I2C address, so they cannot all connect to the same I2C bus. To avoid multiple I2C bus masters in that situation, you can introduce an I2C-controlled analog switch (MAX4572 in Figure 1), which directs the data line to the appropriate SFP module.

Figure 1. Configured as a 1-of-14 multiplexer, this analog switch (IC1) enables a single I²C bus master to address up to 14 SFP modules

Figure 1. Configured as a 1-of-14 multiplexer, this analog switch (IC1) enables a single I2C bus master to address up to 14 SFP modules

IC1, which includes six SPDT switches and two SPST switches. In the MAX4572 each arm of the SPST switches is independently controllable; it is configured as a 1-of-14 multiplexer with an I2C address of 68h. All its COM lines connect to the bus master's SDA line, and each NO output can connect to an SFP module (a 1kΩ pull-up resistor at each SFP input ensures that the undriven SDA lines remain high). The SFP modules retain their addresses, in the range A0h to A2h.