Maynuo  M9812 USB to Serial in DB9 Case - Page 1
Forum section: electronics / projects

author: briandorey
author_rank: Regular Contributor
timestamp: October 19, 2014, 11:34:06 am
content: We got a new Maynuo M9812 a few weeks ago and decided to build our own isolated usb to serial interface so we can use the DC load with the suppliers software for logging battery discharge rates and other projects.The interface fits inside a DB9 serial case and uses a FTDI FT230X USB to UART chip and ISO7421 digital isolator from TI.The isolation gap between the two grounds is not as wide as I would have liked but it should provide good isolation for the voltages that the electronic load will be used for.There are more photos on my blog athttp://briandorey.com/post/maynuo-m9812-serial-to-usb-adapter.aspx

author: ovnr
author_rank: Frequent Contributor
timestamp: October 19, 2014, 11:38:03 am
content: Nice. Recently did one myself, but used a FTDI cable (USB<->UART molded into the plug), and just deadbugged a digital isolator inside the DB9 connector.I'd personally never let a chip bridge the isolation barrier like that, though (FT230X on the back). But, well, if you're not going to shove a lot of voltage into the load it's fine.

author: briandorey
author_rank: Regular Contributor
timestamp: October 19, 2014, 01:10:26 pm
content: Quote from: ovnr on October 19, 2014, 11:38:03 amWe are only planning to use up to 24 volts on the load so the isolation barrier should be ok at these voltages. We will make a larger gap on a new version if we need to higher voltages on the load.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: October 19, 2014, 04:46:17 pm
content: The schematic seems to indicate that the two grounds are bridged. If this is actually how the board is layed out, the two sides are not actually isolated. A ground fault may cause large currents to go through the ground conductor, possible to the point of fusing it somewhere along the line.Another thing to keep in mind is that your circuit will not work if connected to a real RS232 port. While the electronic load only uses 5 logic on its DB9 port, a real RS232 port can have up to +/- 25 V on any of its IO pins with respect to its ground, which is far beyond the ISO7421's absolute maximum ratings. Connecting it to a real RS232 port will likely instantly bust that side of the isolation chip.

author: ovnr
author_rank: Frequent Contributor
timestamp: October 19, 2014, 09:30:40 pm
content: Quote from: nitro2k01 on October 19, 2014, 04:46:17 pmThe Maynuo serial ports aren't normal ports; they're TTL with a +5V power pin, so you don't need to have an isolated powersupply.And yes, the schematic does seem horribly wrong with regards to grounding; I hope it's just an oversight. Right?

author: nitro2k01
author_rank: Frequent Contributor
timestamp: October 19, 2014, 09:52:23 pm
content: Quote from: ovnr on October 19, 2014, 09:30:40 pmThat's what I was referring to, even if I somehow forgot typing V after the 5. The warning was about using the home-built adapter withactualRS232 devices.

author: briandorey
author_rank: Regular Contributor
timestamp: October 19, 2014, 10:17:10 pm
content: Thanks for spotting the mistake on the schematic. I have uploaded the fixed one tohttp://briandorey.com/docs/Isolated-USB-to-UART/isolated-usb-to-uart-schematic.pdfThe PCB does not have the joined ground issue.