Inside the Beckhoff BK5200 – the design of an industrial DeviceNet-to-CAN coupler

Today we’re taking a look inside a piece of industrial automation equipment. The BK5200, manufactured by Beckhoff, is a so-called DeviceNet® Bus Coupler and is used to connect modular input/output (I/O) terminals to the main communication bus. DeviceNet® is a protocol based on the tried-and-tested CAN physical layer; it is used in machine control systems and enables reliable data exchange between PLCs and remote sensors or actuators.

On the front of the unit, there is a block of six LEDs indicating operating status and any errors (these are diagnostic LEDs: I/O RUN, I/O ERR for the internal bus, and statuses for the DeviceNet network), a five-pin DeviceNet bus connector, and an 8-position DIP switch concealed behind a flap, used to set the network address (MAC ID) and baud rate (baud rate).

The module supports up to 64 terminals connected via K-Bus, as shown in the diagram from the documentation:

I will attach the full manual in PDF format at the end of this thread.

On the rear, there is a rating plate showing, amongst other things, the required supply voltage (24 V), the supported baud rates (125/250/500 kBaud), the exact model (BK5200) and the manufacturer’s details (Beckhoff Industrie Elektronik).

The contacts visible on the right-hand side in the photograph form the internal K-Bus interface (six gold-plated pads), through which the device communicates with the connected input/output modules. Spring-loaded contacts for supplying power to the modules are also visible.

Now let’s take a look inside. The first thing that catches the eye is the power supply section (a multi-voltage switching regulator), with the LTC1149CS chip visible in the centre (a high-efficiency step-down converter controller from Linear Technology) and the characteristic three toroidal chokes and yellow tantalum capacitors.

Dismantling the power supply section:

This brings us to the second board, namely the main (logic) board. On it, you can see a large memory chip with a ‘5200 BF’ label – could this be the firmware? – an Epson SG-615PH (40 MHz) and standard logic chips, such as 74HC14D inverters and SN74LS00 NAND gates. The absence of a large processor on this side of the PCB suggests that the circuit’s main microcontroller is located on the underside.

We continue to dismantle the converters:

It turns out there are three PCBs inside:

This reveals the actual ‘logic’ board, densely covered with various surface-mounted integrated circuits:

Here we can see the true heart of the entire device. Taking centre stage is the 16-bit Siemens SAB-C165-LM microcontroller (a ROM-less processor based on the high-performance C166 architecture). It communicates via an extensive bus with a dedicated CAN controller – the SJA1000T chip from Philips (NXP), equipped with its own 16 MHz resonator (a classic choice for hardware-based DeviceNet support). We can also see here two Paradigm PDM41256SA15SO SRAM chips (each with a capacity of 32 KB, giving a total RAM capacity of 64 KB for working memory) and an Atmel AT28C256 EEPROM (with a capacity of 32 KB), in which the device most likely stores its non-volatile configuration. All the chips on the bus are linked together by a CPLD from the MAX family – the Altera EPM7032LC44-15.

Interestingly, the device must have been through a lot, as we can see a truly ‘hacker-style’ modification here, with a through-hole resistor soldered onto the surface of the Altera chip’s pin. I wonder whether someone repaired it, or whether this is actually a factory fix following a batch that left production with a fault.


There is also a small board with side contacts, marked with the symbol BK5005D:

It turns out that this small board is not only a physical interface for plugging in K-Bus modules, but above all a complete interface for the external DeviceNet bus. On the reverse side, there is a 5-pin orange Fieldbus connector and an 8-position DIP switch (accessible from the front panel of the enclosure). Communication with the outside world is provided by a Philips PCA82C250 CAN transceiver, whilst a dedicated circuit ensures full galvanic isolation of the network: high-speed HCPL-0601 optocouplers (labelled ‘601’) for signal isolation and a low-power DC/DC converter based on the Linear Technology LT1111 chip, working in conjunction with a dedicated choke. This module plugs into the main logic board at right angles.

In summary, the Beckhoff BK5200 bus coupler is a device based on a 16-bit Siemens microcontroller (C166 family) working in conjunction with an Altera CPLD logic chip. The hardware resources are supplemented by a Flash memory module in a socket (presumably for firmware), 64 KB of SRAM and 32 KB of non-volatile EEPROM for configuration. The power supply section is based on a switching converter using a Linear Technology chip, whilst DeviceNet communication is handled by a Philips CAN controller supported by a galvanic opto-isolator.

Have you worked with modules of this type? How would you rate their reliability, and what do you consider to be their strengths and weaknesses?