RoadTest: EVAL-CN0313-SDPZ Evaluation Board
Evaluation Type: Evaluation Boards
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: I have already installed a few RS-485EVALBOARD1 (Farnell:1871188) in the field without problems and seeing as they look to be made out of the same parts I am quite happy with this solution.
What were the biggest problems encountered?: Finding sources to reliably cause interference on the RS-485 bus is not easy. Also I needed a source and sink for data to analyze transmission errors.
When I entered for this RoadTest I didn't really think I would get chosen so I wasn't prepared when the package arrived. Like always everything was nicely packed and all the important bits seem to be included. For some reason I always have to pay tax on these review items and with the handling fees this board was actually quite expensive. I have used in the past very similar protection boards on actual projects and these cost somwhere around 10€ whilst this board is priced at over 90€ at Farnell. Granted this board also has the RS-485 transceivers onboard but I don't see how this would justify a price that high. All in all the board looks nice and has three completely separate RS-485 transceivers with different protection schemes attached. There is also a board to board connector for some (probably expensive) processor attachment to fully test this boards capabilities I would assume.
The board is very nicely marked and it is easy to see the different protection schemes used. With the board oriented so the silkscreen is readable it seems that the protection circuits start with the cheapest/weakest at the top and growing more expensive/robust going down.
The first circuit is just one dual TVS connecting both RS-485 lines to GND. This is what I have used on most of my own projects and to be honest I have yet to find the need for more protection. Sure a really bad impulse would get through but my projects are usually cost senitive enough to just hope that this doesn't happen and if it does it's bound to be rare enough that I can amortizize the cost of replacement. The part number of the TVS used is CDS0T23-SM712 and that is available for around 1€ in small quantities (not in Farnell unfortunately). It is a 7 V dual TVS with a peak surge current of 30 A and is available in SOT-23 footprint. The eval board also shows how to correctly mount such a part with multiple ground vias to limit the inductance and resistance for the surge current. Since this circuit has no series devices on the datalines any impedance in the TVS ground path will show up as higher voltage peaks on the transceiver pins.
The second circuit adds a lot more protection. Any pulse coming in from the bus side would first reach a bidirectional thyristor overvoltage protection device (quite a name there). The part number is TISP4240M3BJR-S and this seems to be available at Farnell too for around 0.5€ in small quantities. There is one such device for each dataline. Next there is an inline protection device on each dataline - a TBU (TBU-CA065-200-WH). These seem to act as nonlinear resistors that sharply increase their resistance when subject to excessive current. In my mind these would offer very good protection as they allow decoupling the transceiver from the bus during faults but they are quite expensive at around 1€ each. After the TBUs there is a TVS just like in the first circuit. I think the application would have to be quite important to warrant this level of protection but if there is room in your budget then why not. The additional protection also uses up quite a lot of board space compared to just the TVS so this migh also have to be taken into account.
The third circuit is quite similar to the second the difference being that instead of the thyristor protection GDTs are used. Gas discharge tubes are like miniature flashover tubes that allow a LOT of current to pass in the presence of high voltage. For the 2038-15-SM-RPLF GDT used on this board the maximum surge current can be upto 10 kA for a short duration. The other big advantage of GDTs is their incredibly low capacitance allowing very fast bus speeds to be used (not just for RS-485). The GDT used on this board is a dual symmetric device costing around 1.5€ so it's not too bad. Compared to the thyristor units the GDT has a higher breakdown voltage. The neat thing about GDTs is that in low light conditions you can actually see the discharge sometimes. I had a demonstration in my university with really big tubes some time ago and they really glowed quite brightly with high voltage DC applied (not what they are meant for tho - transients only).
I really don't know how to properly test this board. What I wanted to do first was to set up two MCUs one sending some data and the other capturing it (sending to computer maybe). Then I would try and zap the lines with different sources of high voltage and see what happens. Well eventually I used two FTDI serial cables (the 3.3 V ones) and used a serial loop testing program. Nothing I had at home had any effect on the data at all. So what I want to do now is buy one of these lighters whit the electric spark and zap this into the bus while the test is running. I hope this will corrupt the data but hopefully the board will surveive as I would like to use it somwhere eventually. It would have been nice to get the other board that goes on the board to board connector too as testing this thing is difficult enough as it is. I have no way of actually measuring what I'm throwing at the bus and I wouldn't dare to connect my scope to this thing wile testing. What I would really have liked to see was a note or hints as to how one would actually test this thing. The materials online provide a nice overview on the different transient conditions that these circuits should protect againts but testing anything like that without a dedicated lab is quite difficult. I tried to use a line transformer to generate the voltage spikes by applying DC for a short wile and then abruptly disconnectiong it to let the inductive kickback do its thing but I have no way of knowing if this actually worked or not. I anyone can give any pointers on what I could use to see the pulses on the RS-485 bus I would be very grateful.
Fate of the board
Since this board is meant to be subjected to dangerous levels of voltage and current during the review I was really afraid of breaking it. I actually wanted something like this for a project I was doing for some local radio amateurs. This board (if I don't break it first) will be part of a system used to remotely steer antennas. The antennas are something like 50m tall and are mounted on motorized turntables. I have built a board that mounts under each tower and controls the motor and uses feedback to report the direction of the antennas. The problem with this is that the RS-485 lines from the towers to the control shack are some 300-500 m long and since the towers are so tall they are prone to get hit by lightning. Most of the lightnings energy wil be dissipated in the grounding of the tower but this will cause a huge pulse on th elong bus lines. The first verison I used had very little protection against bus overvoltage and the isolated RS-485 drivers I used all had broken on the bus side after only a few months of use. To address the problem I used the single channel evaluation boards to protect the transceivers and this seems to have cured the problem. Since the control shack has multiple bus connector I wanted to use this eval board there with different protection schemes on each bus segment. I would leave it there for at least one season and see if there are any problems. If no drivers experience problems then I assume that even just the TVS would be enough for all my needs in the forseeable future. Since Element14 requires a review sooner I first wanted to write something here to stay out of trouble and if all goes according to plan then sometime in the future I may get back to this and update on my experience with this board.