DavidLee97

Thank you for the suggestions. I have tried both of them and still no luck. I'm looking at my signal converter right now, and I'm seeing some echo on the RX line from the command transmitted to the slave. The converter I'm using is supposed to mute the RX line while the master transmits, but it's obviously not working. I'm working on the hardware solution to this issue now. What I still don't understand is that the original NI MODBUS VIs appear to ignore the echo and capture the slave response without an issue. Anything else, including modpoll and modscan command line interfaces, all seem to key on the echo as part of the slave response. Here is the NI MODBUS proof-of-concept. This has worked for me at 300 baud every time, even with the echo characters.

I placed a breakpoint after the Init and before the Read. I was able to verify that the RTS/CTS and DTR/DSR lines are asserted. They appear to be unasserted at some point during the Read operation. Also, when running this same proof-of-concept at 9600 baud, I noticed that the RTS/CTS and DTR/DSR lines remain asserted for the duration of the message (master send and slave response). It is only when I try using it at 300 baud that I run into this issue. I am using LabVIEW 2020 and Plasmionique 1.3.5.

I also observe the same image using a Trendnet TU-S9 USB to Serial converter.

The attached image is the same with or without the modification. This was taken using a Tripp-Lite U209-000-R USB to Serial converter. Signal names are: 0=RXD, 1=TXD, 2=DTR, 3=DSR, 4=RTS, 5=CTS. I have shorted RTS/CTS and DTR/DSR.

I have verified that the stop bit and parity are set correctly. The system consists of a USB to RS232 cable connected to an isolater/converter to connect to the Device Under Test. Once side of the isolater/converter is powered by the DUT, the other is powered by the RTS and DTR lines. This last part is presumed, but appears to be verifiable. I used a scope on the RTS and DTR lines while sending the command using the old NI MODBUS library. I observed the RTS and DTR lines remain ASSERTED during the entire transmission (master send and slave response). When using the Plasmionique library, I observe the RTS and DTR lines assert and clear during the transmission. Is there a way to keep these lines asserted during the entire transmission?

Porter, I am trying to communicate with a device at 300 baud, but I keep getting a Slave ID Mismatch error. I have verified that the slave ID is correct (ID=1). Here is an image of the proof-of-concept code that I've tried (LabVIEW 2020):