For wired networks, differential signaling methods are used to increase immunity to common-mode noise sources. RS422 / RS485 are common interface types for point to point or multidrop networks. However, it is limited to 10 Mbps at 10 m, and only 100 kbps at a maximum length of 1200 m. LVDS can theoretically support up to ~3 Gbps for every short cable length (less than 10m), but has a limited DC offset difference between source and destination, which makes it impractical for applications with large DC voltages. IsoSPI overcomes the DC volage offset by using isolation transformers. But it is limited to 1 Mbps up to 10 meters, and reduces to 500 kbps at the maximum length of 100 m. Therefore, wired networks are often inadequate to handle mid-ranged bandwidths (i.e., in the 100-200 Mbps range) in industrial applications.
The next option to consider is fiber optic networks. Fiber provides immunity to high voltages and large electromagnetic fields and provides electrical isolation between the source and destination. Fiber has typically been used for Ethernet based networks from 1 Gbps and 10 Gbps XAUI, up to 100 / 200 / 400 GE. These networks usually require glass based fiber optics, and costly fiber transceivers, typically in the SFP form factor. The bandwidths provided by these solutions are much higher than needed for most industrial applications. However, there are vendors that provide fiber modules that operate in the 100 – 200 Mbps range and can operation over long distances using plastic based fiber cables, resulting in a lower cost solution.
FPGAs are often needed for these fiber networks to support channel coding methods such as 8b/10b encode/decode. FPGA fabric can be used to implement the channel encoding/decoding and the lower levels of the communication protocol. This can be paired with either an FPGA softcore or hardcore processor to handle the higher level communication protocol. The interface with the fiber transceivers often uses LVDS or LVPECL signaling and can be supported by an FPGA without the need for dedicated high-speed transceivers. Thus, a low cost FPGA can be selected. The FPGA selection is based on the level of software support (softcore or hardcore processor) and the FPGA logic resources needed for lower level protocol implementation and hardware acceleration. Therefore, a broad range of FPGA devices are available based on the system requirements. For example, a simple protocol can be implemented in an Intel Max 10 FPGA with a Nios II softcore processor. For more complex protocols and acceleration of data processing in FPGA fabric an AMD Zynq 7000 FPGA with its hardcore processors can be used.
8b/10b encoding transmits K-codes between communication messages when the network is idle. These K-codes can be used to correctly algin the 10b/8b decoding at the destination. A small state machine can detect in-coming K-codes, and control bit-slip logic to lock to the in-coming K-codes and correctly decode the communication messages. These K-codes provide a convenient method to distribute system status throughout the network. Status such as network initialization, node configuration status, error status, etc. can be assigned different K-codes and inserted between messages to communicate system status through the network.
FPGAs provides flexibility to adapt to the needs of the application. FPGAs provide flexibility for various fiber network topologies, such as star, daisy-chain, or various hybrid topologies. FPGA logic can be used to handle the low level routing of messages and to accelerate data processing tasks to free up softcore or hardcore processor cycles. FPGAs provides the flexibility to provide field updates for new features and changes to the network protocol.
Nuvation has experience with fiber interfaces from low speed 250 Mbps fiber transceivers, up to Fiber Channel and multi-gigabit ethernet systems. Nuvation has experience with both softcore and hardcore FPGAs from various vendors.
Contact Nuvation for your next industrial or fiber based application.
