This article looks at systems that use Analog Devices’ CFG4 (24 V, 2 A) bus power scheme. AD2437 based systems support CFG4. Older AD242x parts as well as parts like the AD2433 are intended for use with CFG0 (9 V, 300 mA) bus power. For Clockworks boards with UTP (2-pin DuraClik) connectors, the tan color indicates 9 V and the red color indicates 24 V.
In this article we’ll look at how to connect a 2 wire A2B output (B port) to a downstream A2B input (A port) that uses Analog Devices RJ45 format for the connection. Why is there a problem? In the 2 wire UTP (Unshielded Twisted Pair) system the bus power is carried on the same wires as the data.
In the RJ45 system (typically using 4 pair CAT5 cable), power and data are on separate lines. To avoid damaging incorrectly connected equipment (e.g. Ethernet or some other system using RJ45), the 24 V is not enabled until the downstream node is verified as being 24 V capable. This is done by placing current limited 5 V power on the A2B data pair; this lower voltage is unlikely to damage non A2B devices and the limited current (typically 50 mA, though Clockworks products allow for up to 100 mA) should not damage other non A2B devices. The upstream node verifies A2B capability via reading an EEPROM connected to the downstream A2B node.
UTP systems assume the downstream node is 24 V capable, which is not true if the downstream device is not an AD2437-based (or similar) board. Hence the use of the two different connector colors on Clockworks hardware to try to lessen the chance of releasing the magic smoke from your hardware.
There are several ways to solve this problem. One rather simple one is to use Clockworks’ AB0403 A2B converter (isolator) modules. Normally most of those converters are to go from 24 V CFG4 systems (UTP, RJ45, or XLR) to 9 V CFG0 (UTP) systems. They provide galvanic isolation, which can be an important consideration in some network topologies. With the galvanic isolation also comes the blocking of the bus power.
The AB0403 module is normally used to connect RJ45 outputs to UTP (2-pin DuraClik tan connector) inputs.
The AB0403 comes with a 24 V to 9 V isolated DC/DC converter so that a downstream CFG0 device (i.e. AD242x based hardware with UTP connection via the 2-pin Molex DuraClik) can be bus powered. If the DC/DC is not installed on the board it is possible to use the module to isolate a CFG4 UTP system from a CFG4 RJ45 system to prevent 24 V from being applied to the A2B signal lines. Essentially it is connected backwards.
Before getting into the details, there is a HUGE caveat: The downstream board should be designed so that once 24 V power is available on the RJ45 it will use that instead of the 5 V from the A2B pair. In terms of minimizing power usage on a sub-node this type of power architecture can be the most efficient use of bus power. All Clockworks designed AD2437/RJ45 boards operate this way; once 24 V bus power is available the 5 V on the A2B pair is not used.
ADI boards do not do this; they require the A2B pair 5 V to always be present. Clockworks boards with RJ45 connectors are designed to use the 24 V if it’s available. We’re going to kick the problem of using ADI EVMs down the road and focus on using Clockworks’ hardware or your own custom design where the hardware uses just the 24 V bus power when it’s available, and the 5 V on the A2B pair only during discovery.
Why is the use of 5 V a problem? The 5 V is generated by the upstream node and it regulates it based on the local ground. With long cables ground on the downstream node may be 2 or 3 volts higher than the ground voltage on the upstream node, meaning less than 3.3V is available at the AD2437 Vin pin. This shows up on a network where downstream nodes draw considerable current. In some cases that current is transient when the 24 V is turned on and the just discovered node’s input capacitors begin to charge. If your system uses minimal current and doesn’t have long cable runs you may never experience this issue. Here at Clockworks we did, and that’s why nodes now always run off the 24 V bus power after discovery.
Here’s an example of how this looks in real life. We’ve added a header to the AB0403 for the 24 V return (GND) connection and are using a current limited bench supply to provide 24 V to the downstream board’s RJ45.
Here’s a close-up of the AB0403 connections.
The DC/DC is not installed as it would be driven backwards in this setup. If you’re interested in the AB0403 without the DC/DC please contact Clockworks before ordering as this is not a normal catalog item at the time of writing. Mostly because this is a little bit of an oddball application and we really don’t want you to fry a bunch of hardware because there’s something different about what you need to do.
There’s another consideration: You need to invert the B port data polarity on the upstream node. Unfortunately there’s no consistency across ADI’s three different connector definitions for the data polarity. The UTP design assumes a crossover cable; an RJ45 system, when using standard CAT5 cable, would not cross over data polarity. However ADI swapped the data polarity relative to the power polarity on the A2B pair. This creates no end of headaches for no reason (Clockworks opinion, but we also happen to be right, but the standard is what it is so whatever).
Remember to set the BINV bit on the B port of the upstream node.
