Other Parts Discussed in Thread: THVD1505,
I'm working on a industrial electronics project, and I want to use RS485 modbus to communicate between nodes.
One idea was to use RJ45 jacks and CAT6e cable to deliver RS485 and power to each slave. There are two possible configurations:
1) A daisy chain where power and data are passed from one slave to another. In this case, the limiting factor is the voltage drop along the whole string.
2) Have all RJ45 ports on the master, with the data lines passing back-and-forth between the master and slave. I would use two twisted pairs in the CAT6e: one from master --> slave and another from slave --> master. This way, power is provided to each slave individually and voltage drop is minimized.
The hub would have a single isolated transceiver, and the slaves would be powered over the field 24V. The topology would still be linear, the stubs would just be the length of diff pair between the RJ45 port and the transceiver. Power and ground would look like a star topology.
If using less than 8 slaves, I would have a small board with a 100 ohm termination resistor. Each slave will be connected with no more than 10m of cable.
Some slaves will not be isolated, and use THVD1505 as a transceiver.
Here's an album with what I was thinking.
With a more standard daisy chain, and without power supplied from the local installation, there can be enough voltage drop along the network and violate the common mode voltage range of the transceiver.
Another concern I had is about powering the slave devices from the field.
The TI RS-485 guide shows the ideal network with isolated transceivers and power provided across the isolation gap. My network would be powered from the field, and the slave is not isolated. What happens when hot-plugging the devices, or if the ground connection is lost? 24V would be way above the common mode input limit of the transceiver, and then the protection diodes on the input will find their own ground through the data lines. This may happen randomly, as some contacts make contact before others.
The THVD24x0 is fault tolerant to +/-70V, but it seems from the datasheet that the fault currents are only limited for the receiver. Does this mean, with a missing ground, that the slaves could power through the driver?
How should hot-plugging devices be handled when power is supplied from the field?
