Electronics > Projects, Designs, and Technical Stuff
'Real' RS232 USB Interface
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Gandalf_Sr:
I have a GPSDO that needs a 'real' (as in -ve and -ve voltages) RS232 and the Prolific cable that I bought causes all kinds of weird issues.  I looked around but real RS232 adapters are not easy to find and many have flakey drivers or fake FTDI chips.

I also use the USB-Serial Geekcreit style PCBs a lot (see picture) but I bought a bunch from Banggood and they all had the same serial number! - I ended up buying FT232 ICs (which cost more than the original PCBs) and replacing the fake ones.

So I came up with this project idea, it's kinda a 3 in 1:
1. Stand-alone replacement for the Geekcreit style board
2. Optional MAX3227 RS232 voltage IC
3. Optional to have 5V isolation of the 5V supply to the MAX3227 with digital isolators in the TX/RX lines (not yet shown on attached schematic)

I'm thinking that 2 & 3 will be on a snappable PCB extension and the isolators will be optional on that PCB.

This design includes a feature to allow use with Arduino where the DTR pin allows an Arduino target to auto-reset when a new Sketch is downloaded.

I will bring all pins out to 0.1" headers, not sure if I'll include a DB9 connector, maybe I'll include as I already have the footprint but if not, I'll have a hi-voltage 0.1" header.

Attached is the very first cut (vn 1a) of the schematic for consideration, let me know if anyone has suggestions, request for changes, errors spotted, etc.  I'm going to place a PCB order in about a week so please give me ideas & input ASAP.

[EDIT] I'm going to share this design with you guys for free use in case you were wondering. If anyone wants a PCB, let me know and can can order extra as needed.

Ted
tooki:
There are companies that sell real RS-232 adapters, such as Tripp Lite, which acquired the excellent Keyspan adapter line.
NiHaoMike:
I remember reading that very precise serial time references depend on low/predictable latency on a flow control line which a USB serial adapter will not do. There are PCIe serial cards available for fairly cheap that will work well for that.
coromonadalix:
You have sp-800 usb rs232 cables, they have real ftdi chips and another chip who provide some isolation, they have voltages levels around 5.5 - 6 volts ??
https://www.adafruit.com/product/18

This cable  sp-880  was the best i've found, and we got one recently at my job, with this one i  left it connected to an mcu and i can use the instrument at the same time, other cables did not work well enough (ground issues)


Would it be simpler to interface a max232 ??  or adding an adum xxxx usb isolator ?
Not to be insulting,  but you have already made solutions ??

App notes
https://www.sparkfun.com/tutorials/215

SparkFun Transceiver Breakout - MAX3232
https://www.sparkfun.com/products/11189

David Hess:
I gave up trying to find USB to RS-232 converters which would work with my various Garmin GPS receivers even when the signal levels were correct.  My ultimate solution was to install a PCI or PCIe RS-232 adapter into my desktop computer and it just worked.
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