RS232 signal from SEEED UART interface corrupted when connected to serial device

[IPrin.1]

Hi

I am currently controlling a Fluke serial device over RS232 from a laptop and python script.  I simply send requests for information which is returned and logged in my laptop.  I'm using a Startech RS232 to USB lead incorporating the Prolific - PL2303RA chipset. 

This is a simple setup but obviously requires a laptop to function so I have moved to an STM32F7 board in an attempt to separate from the PC.

I'm writing the same command from the ST board as I was sending from Python, from UART to a Seeed RS232 adapter.  Scoping signals at this point shows them to be ok, whilst not connected to any serial device, yellow being UART and blue, RS232.

However, once I connect to my Fluke device the signals are corrupted to the point if illegibility:

I have reproduced the behaviour using a second UART -> RS232 interface and also switched out the Fluke device for another one from a different manufacturer, but no change in behaviour.

The Seeed interface can be found at 103990363 Seeed Studio | Mouser United Kingdom and uses the MAX3232 chip, whilst the Mikroe one is at MIKROE-222 Mikroe | Mouser United Kingdom and uses the MAX232 chip.  There is a slight difference with how they arrange the capacitors which has some affect on their behaviour.

I'd appreciate any input regarding overcoming this final step of connecting to a serial device as I have hit a wall.  Happy to supply any further information that may help find a solution.

Many thanks

Ian

[Andrew Neil]

Do you have a good ground connection between your SMT32 board, adaptor, and the Fluke?

Please show a schematic.

Some good, clear photos may also help.

Any specifications for the "Fluke serial device" ?

[IPrin.1]

Hi Andrew

Thanks for getting back so quickly, I am utilising UART5 as highlighted below and taking 5V and GDN to the Seeed board.  Grounding is commoned up from the STM32 through the Seeed interface and into the serial leads, I believe it is sufficient.  Any scoping has been done with connection to this ground.

I hope the image below is sufficient to show what I am doing, I'm using a serial splitter to scope the output of the RS232 interface.

The Fluke device is a 6102 micro bath, I have no definitive spec of it's internals unfortunately beyond a reply from their technical support saying they use roughly 2µF capacitors in relation to their MAX232 serial setup.

The Seeed chip uses 0.1µF, could this potentially be an issue?

Ian

[IPrin.1]

Hi Andrew

Do you need any further information regarding my question, unfortunately I cannot get anymore useful info regarding the Fluke 6102.  The other unit I have tried, which behaves in almost the same way is a WIKA CPC 2000, I am trying to get further tech specs on that.

Best regards

Ian

[Andrew Neil]

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@IPrin.1 wrote:

Grounding is commoned up from the STM32 through the Seeed interface and into the serial leads, I believe it is sufficient

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So how does that differ from the working case with the laptop?

[IPrin.1]

Hi

Apologies for not responding yesterday I was away from work.

The grounding is the same between both cases, all USB leads are grounded correctly, as is the RS232 - USB adapter in the working case.  This is the same for the non-working case, grounding is continous through the leads, ST boards and serial leads.

Ian

[Andrew Neil]

I'm out of ideas, then.

[IPrin.1]

No problem, thanks for looking into this.  If I find a solution I will post it here.

Ian