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Way to properly simulate a Modbus RTU device

Created by: AutoGen_4044833 at: 2/28/2023 1:17 PM (5 Replies)

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2/28/2023 1:17 PM	Rate (0)
AutoGen_4044833 Posts: 8 Rating: (0)	Hello, I'm getting mad... I'm trying to validate a piece of program which aims to communicate with a non-Siemens Modbus RTU device. Problem is, such device has not be delivered yet at my office so I would like to simulate it, but I don't know how. The piece of program I need to test is running on a ET200SP CPU 1512SP-1 PN, the RTU connection being made with a CM PtP card. This is supposed to be a 2-wire RS485 bus with a lone slave, numbered 1. That slave is supposed to be passive, and asked for current values periodically by my piece of program. I tried : * Using a spare ET200SP CPU, trying to copy paste the "slave" part from the example program available on the SiePortal. This sample program target S7-1200 while my CPU is a S7-1500, but i don't think this is a big deal. https://support.industry.siemens.com/cs/fr/en/view/47756141 * Raspberry Pi or Arduino boards : these support Modbus RTU only through a separate "shield board" i don't own so I gave up. * USB-to-serial wire, used with the software "KScada Modbus Doctor" : the software doesn't allow me to just set some registers values and wait for requests. So I try the command "write", but it ends up with "time out" error messages. Do you have any other ideas of what I could try? A sofware, an TIA example project or whatever... I really need to have a slave simulator for which I have no doubts about, so if my piece of program really can't get any values, I can reasonnably assume the problem comes from it... Many thanks!
AutoGen_4044833 Posts: 8 Rating: (0)	Last edited by: Jen_Moderator at: 03/01/2023 07:54:47 Link optimized.
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3/1/2023 7:32 AM	Rate (0)
Duris_I Diamond Expert Joined: 4/28/2015 Last visit: 11/27/2025 Posts: 10970 Rating: (843)	Hello, this is little bit problematic because no SW can simulate real device behaviour as is but for test of PLC functionality is any SW on PC recommended. All depends also to connection link to PLC so serial or ETH link is expected use with SW simulator as real device will be connected. I like SW from www.simplymodbus.ca but you can easy look at Topics in Communication Conference for other SWs what users use.
	With best regards... Ivan Duris Flight: OMICO
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3/1/2023 11:10 AM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 1/6/2026 Posts: 1561 Rating: (540)	The usual way is to employ a simulator program on a PC. There are many available commercially or free of charge. I have my own developed from some freeware source code on sourceforge.net. It is called mod_rssim and is very good. You can simulate RTU and TCP Modbus devices, and even simulate serial line noise and error conditions (e.g. framing errors, dropped bits). Modbus Simulator Smiffy.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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3/3/2023 8:48 AM	Rate (0)
AutoGen_4044833 Posts: 8 Rating: (0)	Thank you for your reply. ModRSsim is one of the softwares I tried, unfortunately as far as I know, it can't handle RS485, only RS232...
AutoGen_4044833 Posts: 8 Rating: (0)
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3/3/2023 10:05 AM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 1/6/2026 Posts: 1561 Rating: (540)	Software has nothing to do with RS232 / RS485. This is the electrical interface standard. From a Modbus protocol point of view, it matters not what electrical interface standard is as the protocol is the same. I have used Mod_RSsim on RS232 and RS485. No commercial PC is likely to have RS485 directly from any port. You will need some form of convertor. This can be RS232 to RS485 convertor which plugs into the PC serial port (or into a RS232 USB adapter port), or a RS485 USB adapter. Regardless of which option you choose the Mod_RSsim software must be set to RS232.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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This contribution was helpful to 1 thankful Users danw

3/6/2023 9:18 AM	Rate (0)
AutoGen_4044833 Posts: 8 Rating: (0)	Thank you for this clarification. I indeed thought that the Modbus slave number concept was linked to RS485 rather than the Modbus protocol itself... (as long as RS232 is not a bus, just a point to point standard) After further researches, it seems that the USB interface I used only supports RS232 contrary to another one I worked with so far. I ordered a RS485 compatible interface.. Hope it will work better!
AutoGen_4044833 Posts: 8 Rating: (0)
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