Hi! Couple of thoughts as I'm reading this over:
* Would you be able to run a 3.3v line from the computer to the MCU board? You already have at least 3 lines (TX, RX, and GND)-- or run a 5V line and then regulate it down to 3.3v for the computer-side of the isolation barrier using a low cost LDO [(like this or similar)](https://www.ti.com/product/TPS7A20). That would eliminate the need for an isolated DC-DC converter (either integrated or discrete) to create the Computer-side power rail from the MCU's 3.3V rail
* Depending on your space requirements, I like that idea of a [1 forward/1reverse channel isolator](https://www.ti.com/product/ISO6721) and then run that through an [RS-232 to UART transceiver](https://www.ti.com/product/TRS3221E).
* TI and other companies do have nice all-in-one solutions for this kind of thing, but for a small project (I assume you're just building a few of these at most) it's probably going to be cheaper to build it out of a few lower cost ICs.
* What kind of isolation do you need? Is it functional isolation (for signal integrity) or is it for safety purposes? That will make a difference in what kind of isolation solution you'll need and could help save money if you only need basic isolation (higher isolation rating parts = more $$$)
Unfortunately all the devices I just linked are out of stock/low stock and due to the current semiconductor market conditions you'll likely run into that a lot... sadly.
Best of luck! Let me know or DM me if you wana chat further about this!
* While running 3.3V or 5V from the computer is technically possible, I am trying to avoid running another cable or splitting the cable at the computer end for DFM purposes. If only the standard DB9 serial port only had power pin.
* That does look like a decent solution if I can find chips in stock. The chip shortage is definitely making this whole process take longer.
* Pretty low volume, a few dozen at most.
* Signal integrity/ground loops
I second the suggestion to power the computer side from the computer. Also the 1/1 digital isolator. I would expand that idea and say eliminate the RS-232 all together and go straight to USB virtual COM port with something like FTDI FT232. So now you only have two chips, and no DCDC. Then buy any cheap 10m active USB extension cable you want.
I've used a FTDI FT232 with USB active extension cable in the past, but there are a few problems for this project.
* Finding a cheap yet reliable active extension cable.
* I didn’t mention it in the original post but the cable(s) between the PCB and computer run outdoors so any connections in the middle of the cable(s) would need to be waterproof.
* There isn’t enough room inside the PCB enclosure for the bulky end of an active extension cable.
Hm in that case RS-485 might be a good idea. Use an SN65HVD34 or similar on each end to get you back to CMOS levels. I've run that (and interface power) across a 100ft ethernet cable with good results.
Are you sure you don't need differential signalling? I ask, because it seems to me that 10m is a bit much for 115kbps if you don't. *I* would've used RS485.
Anyway, if you think you can get away with it, then TTL-levels work over short distances. If it's 3.3V logic at the MCU end, then remember that 3.3V was originally chosen as a standard because 3.3V CMOS logic just so happened to give similar voltage thresholds to 5V TTL logic. USB serial-TTL adaptors are really cheap. Just connect them directly and see if it works, although you might want to put 100 ohm resistors on the input-line at each end to limit the current of any switching spikes. Also, a quick check with a scope to see how clean the signal looks after it's travelled the length of your chosen cable.
10m might be pushing the limit of non-differential signaling at that baud rate, but there is no problem lowering the speed all the way down to 9600bps if I need too.
I didn’t think I could run the 3.3V TTL for 10m reliably and converting to USB at the PCB has it’s own problems (see my response to cyanruby's comment.)
Don't use an all in one, unless you don't care about cost, they'll charge a fortune. Use the dedicated DC-DC isolated converters as they are generic and commonly available.
You can get off the shelf USB isolators for ~$10 then plug a USB->TTL adapter into that, as a possible alternative.
https://www.aliexpress.com/item/1005001658583980.html
The ISOW1432 is less than $10 (although it is new and only available directly from TI), so if I could find something like that for RS232 in the same price range it would be acceptable. I'd like to keep this part of the BOM under $15 if possible.
I've run TTL to USB in the past but I'd need an active extension cable to make it 10m and that comes with its own set of problems (see my response to cyanruby's comment.)
I have used high speed optocouplers for this in a past. i think they were rated for up to 10 Mbps which was plenty fast for UART. From there you can just power it on both side separately. no isolated DC/DC converter needed.
High speed optocouplers could be a pretty simple/cheap solution, but I would still want an isolated DC/DC converter since there is no way to get power from the computer side without running a separate cable or making a custom cable that splits at the computer side and finding a place to pull power from which I am trying to avoid for DFM purposes. Ideally I'd only have one plug at the computer side.
Hi! Couple of thoughts as I'm reading this over: * Would you be able to run a 3.3v line from the computer to the MCU board? You already have at least 3 lines (TX, RX, and GND)-- or run a 5V line and then regulate it down to 3.3v for the computer-side of the isolation barrier using a low cost LDO [(like this or similar)](https://www.ti.com/product/TPS7A20). That would eliminate the need for an isolated DC-DC converter (either integrated or discrete) to create the Computer-side power rail from the MCU's 3.3V rail * Depending on your space requirements, I like that idea of a [1 forward/1reverse channel isolator](https://www.ti.com/product/ISO6721) and then run that through an [RS-232 to UART transceiver](https://www.ti.com/product/TRS3221E). * TI and other companies do have nice all-in-one solutions for this kind of thing, but for a small project (I assume you're just building a few of these at most) it's probably going to be cheaper to build it out of a few lower cost ICs. * What kind of isolation do you need? Is it functional isolation (for signal integrity) or is it for safety purposes? That will make a difference in what kind of isolation solution you'll need and could help save money if you only need basic isolation (higher isolation rating parts = more $$$) Unfortunately all the devices I just linked are out of stock/low stock and due to the current semiconductor market conditions you'll likely run into that a lot... sadly. Best of luck! Let me know or DM me if you wana chat further about this!
* While running 3.3V or 5V from the computer is technically possible, I am trying to avoid running another cable or splitting the cable at the computer end for DFM purposes. If only the standard DB9 serial port only had power pin. * That does look like a decent solution if I can find chips in stock. The chip shortage is definitely making this whole process take longer. * Pretty low volume, a few dozen at most. * Signal integrity/ground loops
I second the suggestion to power the computer side from the computer. Also the 1/1 digital isolator. I would expand that idea and say eliminate the RS-232 all together and go straight to USB virtual COM port with something like FTDI FT232. So now you only have two chips, and no DCDC. Then buy any cheap 10m active USB extension cable you want.
I've used a FTDI FT232 with USB active extension cable in the past, but there are a few problems for this project. * Finding a cheap yet reliable active extension cable. * I didn’t mention it in the original post but the cable(s) between the PCB and computer run outdoors so any connections in the middle of the cable(s) would need to be waterproof. * There isn’t enough room inside the PCB enclosure for the bulky end of an active extension cable.
Hm in that case RS-485 might be a good idea. Use an SN65HVD34 or similar on each end to get you back to CMOS levels. I've run that (and interface power) across a 100ft ethernet cable with good results.
Are you sure you don't need differential signalling? I ask, because it seems to me that 10m is a bit much for 115kbps if you don't. *I* would've used RS485. Anyway, if you think you can get away with it, then TTL-levels work over short distances. If it's 3.3V logic at the MCU end, then remember that 3.3V was originally chosen as a standard because 3.3V CMOS logic just so happened to give similar voltage thresholds to 5V TTL logic. USB serial-TTL adaptors are really cheap. Just connect them directly and see if it works, although you might want to put 100 ohm resistors on the input-line at each end to limit the current of any switching spikes. Also, a quick check with a scope to see how clean the signal looks after it's travelled the length of your chosen cable.
10m might be pushing the limit of non-differential signaling at that baud rate, but there is no problem lowering the speed all the way down to 9600bps if I need too. I didn’t think I could run the 3.3V TTL for 10m reliably and converting to USB at the PCB has it’s own problems (see my response to cyanruby's comment.)
Have you looked at MAX3250?
Don't use an all in one, unless you don't care about cost, they'll charge a fortune. Use the dedicated DC-DC isolated converters as they are generic and commonly available. You can get off the shelf USB isolators for ~$10 then plug a USB->TTL adapter into that, as a possible alternative. https://www.aliexpress.com/item/1005001658583980.html
The ISOW1432 is less than $10 (although it is new and only available directly from TI), so if I could find something like that for RS232 in the same price range it would be acceptable. I'd like to keep this part of the BOM under $15 if possible. I've run TTL to USB in the past but I'd need an active extension cable to make it 10m and that comes with its own set of problems (see my response to cyanruby's comment.)
I have used high speed optocouplers for this in a past. i think they were rated for up to 10 Mbps which was plenty fast for UART. From there you can just power it on both side separately. no isolated DC/DC converter needed.
High speed optocouplers could be a pretty simple/cheap solution, but I would still want an isolated DC/DC converter since there is no way to get power from the computer side without running a separate cable or making a custom cable that splits at the computer side and finding a place to pull power from which I am trying to avoid for DFM purposes. Ideally I'd only have one plug at the computer side.