Low power industrial serial communication - Page 1
Forum section: electronics / projects

author: tszaboo
author_rank: Super Contributor
timestamp: July 21, 2021, 10:43:45 am
content: Hi. I've been designing with serial interfaces, like CAN and RS485 for a while now.I've integrated RS485 into a recent product, and all went well, however, we have to go the extra mile to shutdown the transceivers (as in, remove power from them), and reduce the idle time between messages, because the communication protocol just takes too much juice from the battery that powers the entire system.We are talking about an industrial system, battery powered, outside and harsh environment, should survive EMI and RFI.I've been thinking about running simple UART, but that's gotta be less reliable than a differential interface.So here is my question: Is there a low power versions of these transceivers that would use much less power, microamps preferably? So it is not necessary to shut them off. Or if one side would wake up, the other side would be able to detect it, and wake up?

author: nali
author_rank: Frequent Contributor
timestamp: July 21, 2021, 11:23:27 am
content: Maxim/Linear SP483 has a shutdown mode which they reckon drops consumption down to 1uA typ 10uA max. But it's 5V only so don't know if it'd suit your battery powered application.

author: 2N3055
author_rank: Super Contributor
timestamp: July 21, 2021, 11:50:40 am
content: LTC1481 ? But still need to be put into shutdown mode to achieve 1 uA consumption..Also SN65HVD37.. There are few of them.Keyword : "rs485 Ultra-low Power Transceiver"It easily achieves your power target. Only thing to worry is receivers wake up when master sends..

author: voltsandjolts
author_rank: Unknown
timestamp: July 21, 2021, 12:25:23 pm
content: Perhaps Renesas ISL32602E et al are worth a look.

author: PeteH
author_rank: Unknown
timestamp: July 21, 2021, 12:38:44 pm
content: What's the required data rate? (Actual bits/sec required on the bus)

author: 2N3055
author_rank: Super Contributor
timestamp: July 21, 2021, 12:52:31 pm
content: Quote from: voltsandjolts on July 21, 2021, 12:25:23 pmThat one seems EOL, but there are other interesting ones from the family..

author: tszaboo
author_rank: Super Contributor
timestamp: July 21, 2021, 12:54:45 pm
content: Quote from: PeteH on July 21, 2021, 12:38:44 pmNot important, infrequent data transfers (every few seconds), less than 100 bytes. What matters is the power used during this.Now its running at 115200 baud.Quote from: voltsandjolts on July 21, 2021, 12:25:23 pmThey are indeed. 1.8V operating voltage is already something that I never heard of, currents look very small. Interestingly, they show a system without termination resistors on Figure 3, I wonder if this is a mistake or intentional.Quote from: nali on July 21, 2021, 11:23:27 amFirst time I see this company. 5V is not interesting.Quote from: 2N3055 on July 21, 2021, 11:50:40 amWell putting it to sleep and waking it up is slightly annoying, limits what you can do in Firmware. Would be preferable if it would wake up by itself.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2021, 01:56:30 pm
content: Nothing prevents using CMOS gates to drive the cable and using AC termination; idle power could be reduced to microamps this way or less. Then the problem becomes finding a suitable low power differential line receiver. 5 microsecond comparators with 10s of microamps of supply current are available but I am not sure how to do better than that.

author: tszaboo
author_rank: Super Contributor
timestamp: July 21, 2021, 03:00:34 pm
content: Quote from: David Hess on July 21, 2021, 01:56:30 pmAnd how do they handle EMI? I am talking about lightning strikes, trains in close proximity, motors and pumps and other highly inductive loads switched on within 10m of the device or the cable?

author: ajb
author_rank: Super Contributor
timestamp: July 21, 2021, 06:25:19 pm
content: How many devices and what sort of distances? Is there one battery with power distributed through the cable to all of the devices, or is each one powered independently?100 bytes every three seconds at 115kbaud is about a 0.3% duty cycle, so powering down the transceiver between messages seems like a better strategy than trying to reduce the active power consumption of the transceivers.If you hold the line active for a couple of character periods at the start of each message you could use a slower and maybe lower power comparator with some hefty low-pass filtering for noise immunity to provide the wakeup signal, and you can also use that as a framing signal since it'll trip the framing error flag in your receiver's UART. The comparator will of course load the bus a bit, but you can probably get away with that given enough resistance at the input and few enough devices or low enough unit load transceivers. Or if you don't want to add extra hardware, maybe make that break signal a bit longer and have the devices periodically wake up and check the state of the bus.A more crude solution, if you have an extra conductor in the cable, would be an open-collector 'bus active' signal that enables the transceiver? Can again be pretty heavily filtered as long as it's asserted a bit before transmission starts. Wouldn't actually need to be open collector if the bus is controlled by one device.

author: PeteH
author_rank: Unknown
timestamp: July 21, 2021, 06:44:54 pm
content: RS-4xx bus with a strong AC coupled termination scheme should be very robust and relatively low power due to the low duty cycles....

author: fcb
author_rank: Super Contributor
timestamp: July 21, 2021, 07:48:20 pm
content: Transformers. Deals with the balancing of the data lines and isolation. You might need to modify the data format if you expect heavy DC data content.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2021, 07:49:34 pm
content: Quote from: tszaboo on July 21, 2021, 03:00:34 pmI would expect to add some series and shunt protection to outputs and inputs in the form of schottky diodes and resistors and maybe some capacitors to meet the same requirements that dedicated RS-485 transmitters and receivers meet.

author: wizard69
author_rank: Super Contributor
timestamp: July 22, 2021, 12:43:11 am
content: Ok I'm going to go in the opposite direction here, why move away from a proven tech and apparently working software for the unknown? Instead address the lack of power. That can mean larger batteries but I'd seriously consider solar. Also what can you do on the microprocessor end to lower power there.Beyond all of that you are running a communications cable to the device why not a couple of more wires to supply power to the unit? You don't even need to run power the full length, if you want the solar supplementation, it could be remotely positioned a bit to grab the light.The way I look at this is that the minute you go to something non standard you own it. With something like RS485 there is likely to be hardware available for decades longer. I just don't see it going away completely anytime soon. There are likely other standardized hardware solutions out there but I don't know of any real comparisons with respect to power usage. Beyond all of that what happens in the future when you or a customer, wants your device to sit on the 485 with another device? You have something working now, adopting the old "if it isn't broken don't fix it" concept might be a good idea.After all of that you have to think about maintenance or long term support.

author: 2N3055
author_rank: Super Contributor
timestamp: July 22, 2021, 06:26:09 am
content: Quote from: tszaboo on July 21, 2021, 12:54:45 pmWell , you can keep standard RS485, together with robust transceivers, and just add separate low power signal detector that will wake up device......Signal is quite strong, no problem to detect it. When sending it. you send wake up packet, than payload. You can even have "Over and out" packet so devices know they can go to sleep. And/ or timeout...

author: tszaboo
author_rank: Super Contributor
timestamp: July 23, 2021, 08:33:16 am
content: Quote from: ajb on July 21, 2021, 06:25:19 pmTypically 2 devices. Distance is only up to 30m.I guess it could be possible to detect if the A-B line voltage is above a certain threshold.I don't really want to add extra wires. These are M8 and M12 connectors, and though 6 pin variants exist, they are less standard.Quote from: wizard69 on July 22, 2021, 12:43:11 amNah, rechargeable batteries are dangerous. They would need to survive a nail penetration test and short circuit test without setting anything else ablaze. But they are also result in a more expensive solution. I can buy a primary battery and do 10 year battery life with wireless sensors easily, I dont see how this would be impossible with wired.Quote from: fcb on July 21, 2021, 07:48:20 pmTransformers and inductors store energy, and they are for all intends and purposes not allowed in these applications of mine.Quote from: PeteH on July 21, 2021, 06:44:54 pmNow that's an interesting idea. I never used AC termination on RS485, though seen it mentioned. I guess, you would need to size the termination to the cable length, but I guess it just need to be sized to the maximum cable length, and shorter ones will have a small overshoot. Its not like I have kilometers of cable (which I've seen with 485)

author: Berni
author_rank: Super Contributor
timestamp: July 23, 2021, 10:48:34 am
content: There are some RS485 transceivers that feature an offset receive threshold, making them see a 0V differential as being a logic 1. So this saves you from the power hungry voltage divider termination that requires a power supply.Another option are CAN transceivers, those see no voltage on the bus as idle and a positive differential voltage as active. There is no reason why you can't just simply send UART data trough a CAN transceiver chip.

author: fcb
author_rank: Super Contributor
timestamp: July 23, 2021, 10:52:35 am
content: Are both devices battery powered? If not, can you power one device from the other via the RS485 bus?

author: tszaboo
author_rank: Super Contributor
timestamp: July 23, 2021, 12:10:51 pm
content: Quote from: fcb on July 23, 2021, 10:52:35 amBoth devices are battery powered, one is powered from the other. Has an LCD, constant power is needed.It's like a remote display for a sensor.Well, at least in one configuration. I want to use this architecture in other devices as well.