A/D to RS485 without a uController - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: September 24, 2018, 07:38:55 pm
content: I have been tasked with the design of a remote voltage measurement board that will communicate with the "Main" controller that has a RS485 interface. Code on the control board side still needs to be written, so fairly flexible re protocol, but it will need to be addressable and only respond when it's supposed to.Obvious way to do the remote end is to use a small uController with an integrated A/D and connect the UART to a RS485 converter. Downsides being need to write the code and then program each remote board.I'm wondering if an addressable RS485 A/D converter or daughter board already exists that I could use?12bit would be fineAnyone come across anything like this?

author: Rerouter
author_rank: Super Contributor
timestamp: September 24, 2018, 08:46:51 pm
content: From what I can see, they exist as modules with a micro and an ADC on them, I would assume they don't exist in the wild as a chip is because of the addressing scheme requirements, as most people roll there own for there purposes rather than follow some defacto (e.g. SAE J1708 / J1587)

author: guenthert
author_rank: Frequent Contributor
timestamp: September 25, 2018, 08:32:23 am
content: Sorry, I must ask why. Small MCUs are so cheap and easy to use that there must be a good reason not to use them.Anyhow, how 'remote' is remote? And you mentioned that the MCU has RS485, but does it mean it has to be used? If not, and if the distance is not too large, a voltage to frequency converter might be an option. That signal can be carried a few meters easily.

author: Jester
author_rank: Frequent Contributor
timestamp: September 25, 2018, 01:19:29 pm
content: Quote from: guenthert on September 25, 2018, 08:32:23 amAs stated"Downsides being; need to write the code and then program each remote board."Remote as in typically 50-500'I did find a few modules that do exactly what I need but they are >$200 a pop, expect we will need 100 in the long run, so I might have to fire up the Microchip IDE.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: September 25, 2018, 02:56:13 pm
content: I am sure somebody on here would provide working code for 1k$ or less.All the boards would have the same code. Program at time of manufacture.100 boards X $10 / board + 1000 = $2000.

author: coppercone2
author_rank: Super Contributor
timestamp: September 25, 2018, 02:59:39 pm
content: Are you looking to get rid of a MCU for reliability/power/quality/security reasons with an ASIC or hard logic, or are you just looking to not do something and don't care how its handled?You can do this with alot of gates an a asynchronous oscillator. With those small form factor logic gates I think it could actually be fairly physically small but its alot of parts.may be easier with parallel output ADC rather then serial.It would be like a industrial i2c

author: max_torque
author_rank: Super Contributor
timestamp: September 25, 2018, 03:23:22 pm
content: I can't believe anyone still thinks it's "easier" to build some frankenstein super complex device out of logic ICs and oscillators and analogue do-dahs, when you can just buy a small uC for sub $1 that'll do everything you want, easily, robustly, and with little time required to develop it?There's a micro in just about everything these days precisely because they are so cheap and easy to use.......

author: coppercone2
author_rank: Super Contributor
timestamp: September 25, 2018, 03:41:08 pm
content: Quote from: max_torque on September 25, 2018, 03:23:22 pmreliability/power/quality/security reasonslike it or not a MCU has 'engineering dead space' in it. anything programmable does. It's undeniably more complicated then other solutions, but the problem is that the bond wire and solder joint reliability compared to the single substrate, but a ASIC would be more reliable then the MCU.If it was bare die on a ceramic it would be quite good since the amount of solder joints would be minimized, but you still have a lot of bond wires. It should also have lower power consumption and EMI profile will be different.It also does not require code to maintain and the programming process requires strict quality control.And of course you can probe it as a part of maintenance to see if there is signal degradation in any stages, and you can check power consumption on subsystems (such as to see if gate drive impedance changed) etc, if designed right with the right manual a technician can know exactly how its aged and such (if its discrete or your ASIC has alot of taps on it).You can make a big ASIC with some pins in critical areas that can hook up to the right kind of DAQ to do a full diagnostic on it. With a MCU at most you can do a memory dump or maybe a few more things if you have special features on it.e Maybe you can design some test codes to 'stress' things that might give you a clue as to its internal condition by small variation but its not really cut and dry. I think you would need to have very intense knowledge of the MCU semiconductor sturcutre to write test programs that might give you ideas on internal condition based on things like jitter, propagation times, etc.FPGA is interesting because you can fuzz it real good but you still can't probe internal stuff on a conventional one. If some 'on configuration' switch matrix is going, only god might be able to predict that failure.

author: ogden
author_rank: Super Contributor
timestamp: September 25, 2018, 04:28:47 pm
content: Quote from: Jester on September 25, 2018, 01:19:29 pmMCU flashing time is not even argument for 100 pieces. If done right, you can flash one small MCU each 10..20 seconds = 180 to 360 per hour. MCU provides flexibility. Fact that you have MCU is good in application-specific solutions. Just pick some MCU you know with ADC that is good for application and start programming. Don't waste time in the forum

author: IDEngineer
author_rank: Super Contributor
timestamp: September 25, 2018, 04:43:41 pm
content: That "Frankenstein" comment was 100% on target. A small MCU with everything you need will be FAR more reliable than a board full of discrete logic. The latter will have far more bond wires (if that's really the issue here) than a single MCU package.If you just desire to design a board-level project that (let's be honest) reinvents what you can instead buy in a single IC package for far less money in far less real estate with far more reliability, go for it. But don't do it for the wrong reasons.Not trying to start an argument, just analyzing the various comments here from the 100K foot level. YMMV.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 25, 2018, 05:05:32 pm
content: Have you looked there?https://www.eeci.com/

author: DaJMasta
author_rank: Super Contributor
timestamp: September 25, 2018, 05:59:38 pm
content: There are ADCs with UART outputs, then you'd just need your line driver for RS485. There are also RS485 to SPI (and other) translator chips.Micros are the easy choice because their availability is broad and you can configure them for almost whatever you want, so they may even be the cheaper option even if using an external ADC, but there are still specialized interface ICs and ADCs that support UART serial output.

author: coppercone2
author_rank: Super Contributor
timestamp: September 25, 2018, 06:28:25 pm
content: Can you show the UART ADCs? I found a few but is they have on board MCU, its not a hard circuit with a pin strap address or something. Thats more like a SOC or just pimped out MCU.http://www.analog.com/media/en/technical-documentation/data-sheets/ADUC816.pdf

author: DaJMasta
author_rank: Super Contributor
timestamp: September 25, 2018, 06:44:06 pm
content: Something like the ADS112U04 would be an option if low sampling rate is fine. In any case, there's a good number with UART outputs on digikey, I imagine there are some choices around.Thinking about it, though, you could also just get a parallel output ADC which are plentiful, then a parallel to rs485 converter which is quite unlikely to be a MCU.

author: coppercone2
author_rank: Super Contributor
timestamp: September 25, 2018, 06:45:01 pm
content: how do you address it? It sounds like it would only work as one.

author: max_torque
author_rank: Super Contributor
timestamp: September 25, 2018, 06:54:47 pm
content: Assuming you pick an off-the-shelf and common microcontroller (your choice, AVR/PiC/ARM whatever) then you must be pretty dam good at building high reliability electronics to even start considering the reliability of the actual micro itself in your design! There are litterally billions of these parts made every year, and the failure rate is both tiny, but critically, very well documented. The rest of the circuit you build will undoubtedly be the source of 99.9999% of any reliability issues.........I've engineered life rated electronics and the micro is so far down my list of "uh-oh" it's doesn't even register really!And as to having to manage software, well, if you "code it in hardware" then you have to manage the hardware, which is even harder, and more likely to change (for example, that 10uF timing cap, what if it goes obsolete, and you need to change to another make? or that precision 10k resistor, or about a million other analogue parts that MATTER when you do everything yourself from scratch.And even better, if you take the time to write the code for a good RS-485 "node" using a micro, it's trivial to then make a different sort of sensor or actuator, re-using all the knowledge and learning you already have. Do it with descrete's and you're starting again from scratch......

author: coppercone2
author_rank: Super Contributor
timestamp: September 25, 2018, 06:56:47 pm
content: no one said it was easy or worth doing in most applications but the option always exists and should be known. The MCU is chosen just because of cost and ease of use, otherwise everything would be custom tailored.Plenty of higher end people end up making ASICs for digital functions that can be done with a MCU for all sorts of reasons, usually power consumption.Most of current engineering culture might rate some kind of more discrete solution for maybe a mission to another universe or something, you are right about that. Like the second futuama movie lol. I am pretty sure most bosses would encourage you to hang yourself if you propose doing that with their precious money.

author: Jester
author_rank: Frequent Contributor
timestamp: September 26, 2018, 10:35:05 pm
content: Quote from: SiliconWizard on September 25, 2018, 05:05:32 pmThanks for the link, price is right. In the end, I'm probably going to use a PIC or similar and write some code.Cost of programming a part in production is a lot more than the time to do it, especially when the company has ISO system etc. Code needs to be written, tested and reviewed, programming procedures need to be written etc. It all adds up $$$, so if there was a magic part I was going to simplify the process, not this time I guess.Thanks to all that responded.