topic Re: Serial port with no stop bit in LabVIEW

Serial port with no stop bit

TristanSullivan — Tue, 29 Oct 2019 16:07:32 GMT

I need to communicate with a MODBUS slave through RS232, and the documentation for the slave specifies that there should be no stop bit. As far as I can tell, Labivew doesn't let you configure a serial port with no stop bit. Is there some way to do this?

Re: Serial port with no stop bit

cbutcher — Sat, 01 Mar 2025 00:32:04 GMT

It might be worthwhile taking a look at one of the MODBUS libraries available for LabVIEW. They probably have built-in handling for several MODBUS-specific issues.

One library is built in if you have either the "DSC Module or Real-Time Module", according to the help files.

Others include LabVIEW-Modbus-API or the older MODBUS Library for LabVIEW.

The first of those links provides an overview of some of the others, copied below (original author is smithd):

Modbus is a commonly used protocol for industrial communication. At present, National Instruments provides four options for Modbus connectivity.

(1) Modbus I/O server, available through the LabVIEW Real-Time Module or the LabVIEW Datalogging and Supervisory Control (DSC) Module.which provides a high-level API for accessing Modbus data. Learn more at Connect LabVIEW to Any PLC With Modbus

(2) A free Modbus library available for download which is very low-level, and provides control over Modbus not offered by I/O servers at the expense of the connection management and request scripting offered by that higher-level API. **This Library is Unsupported**

(3) This NI Labs Modbus API release which is intended as a replacement and expansion of the free Modbus library functionality. **This Library is Unsupported**

(4) The Modbus API available through the LabVIEW Real-Time Module or the LabVIEW Datalogging and Supervisory Control (DSC) Module. This is a version of the NI Labs Modbus API which is now supported and maintained by National Instruments R&D.

In addition, an NI Alliance Partner SAPHIR has two released LabVIEW add-on products called ModBusVIEW over TCP and ModBusVIEW over Serial available on the LabVIEW Tools Network.

Re: Serial port with no stop bit

GerdW — Tue, 29 Oct 2019 16:20:08 GMT

Hi Tristan,

mind to name that slave and attach (or link to) the manual?

Re: Serial port with no stop bit

TristanSullivan — Tue, 29 Oct 2019 16:24:21 GMT

Thank you for your reply. It's a Lytron recirculating chiller, manual attached. The relevant information is on page 28.

Re: Serial port with no stop bit

JÞB — Tue, 29 Oct 2019 16:33:21 GMT

@GerdW wrote:

Hi Tristan,

mind to name that slave and attach (or link to) the manual?

Oh, no Problem! I'll just shake the reliable 8-Ball and

~~~~Outlook uncertain~~~~

OK, I cross posted with the OP ( How did I miss That! Silly 8-Ball needs to be re calibrated)

8,none,&1 is the default for RS-232 but you seem to be describing a MODBUS-RTU via ethernet? What is the PHY layer?

Re: Serial port with no stop bit

TristanSullivan — Tue, 29 Oct 2019 16:37:29 GMT

Thanks for the reply. I'm using RS232, not ethernet. My problem is with the number of stop bits, which can only be 1, 1.5, or 2, as far as I can tell. Am I missing something?

Re: Serial port with no stop bit

RavensFan — Tue, 29 Oct 2019 16:51:59 GMT

What you are missing is that the manual is wrong and needs editing. You can't have a serial communication without a stop bit.

8-n-1 means 8 data bits, No parity, 1 stop bit.

Re: Serial port with no stop bit

Ken_Naylor — Tue, 29 Oct 2019 16:55:14 GMT

Could the manual have a typo?

19.2,N,8,1

would normally mean Baud=19.2, No Parity, 8 Data, 1 stop

I have never come across No stop bit!

Re: Serial port with no stop bit

TristanSullivan — Tue, 29 Oct 2019 17:01:56 GMT

Thank you! That worked.

Re: Serial port with no stop bit

billko — Tue, 29 Oct 2019 18:42:04 GMT

Hint: you can have mark more than one replay as a solution. I can see at least one other post that deserves a solution - and it came before the one you marked.

Re: Serial port with no stop bit

RavensFan — Tue, 29 Oct 2019 20:47:53 GMT

Another problem with that manual on page 28 is that it says pin 2 is RS-232 positive and pin 3 is RS-232 negative.

NO SUCH THING!

One is transmit the other is receive, and both are relative to pin 5 ground.