Matthew, please post a message here when you have a new version of Mango that works with a "Serial Device Server" option for Modbus serial RTU protocol. That would help put Mango on the same level as Modbus master software like Modpoll and Modscan32 (as far as Modbus protocol support). Then with all of Mango's other features, it would leave them in the dust!

Just an FYI for anyone wanting to use an SDS or SDA with Modbus serial RTU protocol: you usually need to tweak the adapter's settings to allow for the way that the protocol determines the end of a packet (i.e. with a silent interval). For the Moxa NPort 5130, I configured it for a "Packing length" of 256 and a "Force Transmit" of 5 milliseconds. This also makes analyzing a communications session with Wireshark easier.

Of course the SDA approach doesn't support multiple concurrent sessions like Modbus TCP/IP can, but for a lot of applications that's beside the point.

Sorry, I forgot to mention the slave address is 127 (7F hex). Here's a hex dump of the command and reply I just captured with my C# Modbus master program that reads 2 holding registers at address 0 (or addresses 40001 and 40002 in Modbus terminology):

8 bytes sent: 7F 03 00 00 00 02 CE 15
9 bytes rcvd: 7F 03 04 00 00 00 00 64 34

Be sure to use a long enough reply timeout to allow for extra timing delays due the to the Internet. I use a 3 second timeout. Also, the Windows TCP/IP sockets API has some options to help make sure the packet doesn't get fragmented at the Ethernet layer e.g. their's a flag called ClientSocket.NoDelay that should be set true. For Linux, there should be a similar flag.

BTW, not very many Modbus slaves support the function code for reading exception data that Mango uses for automatically identifying the slaves on an RS485 drop. A more universal way is to just read some register (40001 is a common one). If the slave has a register at that address, it will give you a normal response. If not, it will respond with "illegal address" exception. Either way, you've discovered that there's a slave at that slave address.

I think what ModPoll calls "encapsulated RTU" is what Modscan32 calls a "remote telnet server" connection, which just treats the TCP/IP socket like a serial port. That is, just sendng plain Modbus RTU packets via the socket without the extra 6-byte header that Modbus TCP/IP requires.

It would be a cool feature for Mango to support that connection type so folks can use cheaper serial device servers with Mango instead of pricey Modbus Ethernet gateways.

I can probably connect my Moxa to the Internet for you to test with later this week if David can't make his available.

I should have said Mango works fine with a Lantronix Intellibox Modbus Gateway with RTU slaves but also works fine with the Moxa 5130 (a basic Ethernet/RS485 SDA that is not Modbus-aware) for those Modbus slaves that can handle the 6-byte Modbus TCP-RTU header forwarded over RS485.

Does anybody know of a USB/RS485 adapter with a reliable Ubuntu device driver that works with Mango? Also, any tips or hyperlinks on how to install the driver to make it work with Mango would be a big help. I know how RS485 works but I just need a few pointers on getting a USB/RS485 adapter to work dependably on Ubuntu.

I saw Mango described in the latest Modbus.org newsletter and I'm glad I checked it out -- it works well with Modbus-TCP/RTU to a single slave via a Moxa NPort 5130 Ethernet/RS485 adapter. Now if I can just find a USB/RS485 adapter that works as well!