SFP modules are interchangeable and even ‘hot swappable’, allowing the end user to change the IO configuration of products in the field – easily changing equipment over from coax interfaces, for example, to fiber, HDMI, or even ST 2110 / ST 2022-6. End users can also select different processing options – reclocker or no reclocker on coax parts, or various CWDM wavelengths on fiber modules, for example.

The process for incorporating SFP cages into a video product is relatively straightforward, but there are certain design aspects that need to be considered. This article is intended to provide guidelines for a successful integration.

SFP Type

Several SFP types exist on the market, but for this discussion we shall concentrate on “SFP+” which is widely used in Video applications. The original “SFP” specification was limited in the data rates that it could handle and is now mostly relegated to 1GbE networking applications. The original SFP could still be used for 3G SDI, but the newer SFP+ is recommended for new designs, and works very well for all SDI signals up to 12G. Not only do the newer SFP+ modules support higher data rates, the SFP+ cage can accommodate modules with higher power consumption.

The SFP Cage

The cage itself can be sourced from various manufacturers. It is composed of a metal body with a host connector for the electrical interface. The metal body is the same for SFP/SFP+, but SFP+ host connectors have better signal handling and appropriate impedance matching for the higher bitrates¹.

The metal body has pins used to connect it to the host PC board, with EMI shielding surrounding the pass-through opening in the host product’s enclosure. The pins can be soldered or pressfit type, but soldered connections to the PC board are strongly recommended for video SFP’s, to support the weight for coaxial and HDMI cables.

An important consideration when choosing an SFP cage is the retainer clip. This is a small triangle-shaped hole on a spring-tab. This retainer clip needs to be carefully studied when choosing the cage for your product, to not only hold the SFP securely in place … but also to release properly when the SFP latch is activated to pull the module out. The most frequent cage issues we’ve seen usually have to do with poorly functioning retainer clips. An inadequate mechanical design may create other problems, such as poor signal connection or high bit error rate.

Cages are also available with an integrated heatsink. This is advisable, particularly if there is limited airflow or higher ambient temperatures within the host product housing. Video SFP’s may consume more power than data SFP’s in general, particularly when processing modules are used which host ST 2110 and ST 2022-6 gateway applications.

Connector Pinout: MSA Or Non-MSA

The SFP form factor was initially developed specifically for the data communications industry, to carry IP based traffic. MSA stands for “Multi-Source Agreement”, and it refers to the detailed set of specifications that various Transceiver manufacturers agreed to for ensuring interoperability between modules and host equipment².

IP traffic, however, – by its very nature – is always ‘bi-directional’. The electrical pinout for the SFP module was strictly defined for bi-directional signals (100 Ω differential pair TX and a 100 Ω differential pair RX) with no allowance for any variation.

When innovative companies first came up with the idea of using SFP modules for SDI based video signals, rather than IP data, they realized that many applications within a video broadcast facility are not bi-directional … there are many situations with a need for two signals going in one direction. (Consider the requirement for a video camera, which probably has at least two video outputs but no inputs … or a video monitor, which probably has at least two video inputs but no outputs).

This led to the development of an alternate connector pinout, which is now known as “Non-MSA”. When used for video signals, the MSA pinout only supports video which is in a Transmit/Receive configuration. Non-MSA adds support for dual transmitters and dual receivers.

The MSA ‘pinout specification’ is simply an agreement between manufacturers, it is not recognized or formalized by any international standards body. Similarly, the “Non-MSA” pinout has also become a de-facto standard on its own and all Non-MSA modules are interoperable between vendors – but Non-MSA modules cannot be plugged into a cage wired for MSA. Non-MSA cages, however, can be configured to accommodate both module types.

If the SFP cage is strictly intended to be used with SDI (or MADI) signals on the host connector, then Embrionix recommends designing the host PC board to use the Non-MSA pinout for greater flexibility.

(Most Embrionix SFP modules are available in both MSA and Non-MSA pinout, where applicable, so products designed with MSA pinout are still fully supported).

For technical details on designing the host equipment electrical interface for supporting any combination of 2T/2R/RT signals, please see the Embrionix Application Note “Generic Connector Pin Outs To Support The Three Different Non-MSA SFP“

Module Control: The I2C Bus

All SFP modules use I2C (Inter-Integrated Circuit) for serial communication from the host, to provide monitoring and configuration. I2C was chosen due to the fact that it only requires 2 pins for communication, and is inexpensive to implement. The downside is that the data throughput is quite slow compared to other control options such as SPI.

I2C uses bidirectional open-collector/open-drain lines: serial data line (SDA) and serial clock line (SCL), pulled up with resistors. The host equipment will need an I2C controller interface, and some programming work will be required even for basic integrations.

Through the I2C bus, the host system can access register spaces A0h and A2h within the SFP module. Each register is mapped to an address and may have read-only information (for device ID, alarms, or status monitoring, for example) or it may be a read/write register for configuring specific settings in the module.

By reading registers such as the “Vendor Name” and “Vendor Part Number”, the host equipment can be programmed to accept or reject certain modules – for example, only accepting modules that have been tested and are known to be compatible with the host system.

Note: When buying SFP modules from Embrionix, equipment manufacturers can custom ID their parts. With customized modules, the exterior label will have the equipment manufacturers logo and part number – and also the vendor name and part number is programmed in the registers.

Care should be taken when routing the I2C bus. Capacitance should be minimized and limited to 400pF (per I2C specification) and only one SFP should be directly connected to the I2C bus controller. Otherwise, contention will appear since SFPs all have common I2C physical bus addresses. Physical address A0h contains the EEPROM and A2h contains the warnings and alarms. If multiple SFPs needs to be controlled by a single I2C bus controller, an I2C Switch or Address Translator could be used on the host board to avoid bus contention.

Conclusion

Incorporating SFP cages into professional video products bring undeniable advantages. This article has shown that the process to do so is straightforward but comes with specific design considerations. Although Information Technology SFPs are really similar to Video SFPs, the latter comes with some differences, namely regarding power budget, heat dissipation, bit rates, and host pinout (if choosing to go with Non-MSA option). These considerations need to be properly evaluated for a successful design.

For additional technical details on the circuitry requirements for the rest of the pins on the host connector, please refer to the Application Note “How To Design With Video SFP“.

¹  The specification SFF-8431 includes measurements to ensure the compatibility between the host PC board and the SFP+ modules.

²  Since its introduction, a more recent version of SFP MSA Transceiver multi-source agreement was released. It has been renamed SFF-8071 and is managed by the SNIA.