RS-485 Multi-Drop
Does RS-485 multi-drop functionality work to be able to read multiple devices across a single serial connection? If not, can this feature be added? Or, are there any proven workarounds (e.g. a multi-port serial to single USB adapter, or another method)?
Dear Matt,
thank you for reaching out to us. The Serial COM module supports RS232 and compatible protocols (RS485). The RS-485 is a physical layer interface. Every node on the network can generally listen at the same time. However, only one node can talk at any given time. If more than one node attempts to talk at once, the message will be garbled. So, a higher level protocol is necessary to ensure that every node will get a chance to talk and that its message will get through ungarbled. We do not have experience with multi-drop functionality. We have a 30 day trial license which you can use to test the Serial COM module for the RS-485 multi-drop functionality.
Kind regards.
Dewesoft does not support multi-drop. The software does not have a way to select an individual node number unless the option is hidden somewhere. I am using a pcie serial converter with rs232, rs485, rs422 but a multi-port usb converter would work as well.
Gary
I am not an export on this, but looks like some rs485 devices just send the node number as prefix (dewesoft can can do this) to a standard command but others use the 9th bit. How the node information is formatted, I don’t believe is part of the rs485 protocol, as it just describes the physical layer. So this would be defined by each manufacturer. I just got in some rs485 devices that support multi-drop that will be hooked up to dewesoft, but I have not had a chance to read through the documentation yet.
Thanks. The devices that I'd like to use are Mensor (Wika) pressure transducers.
Per the Mensor manual, the command to return a pressure reading is:
#X?<cr>, where X is the node address (single number or letter), or a wildcard: *
and the return string is:
X<sp>nnnnnnnnnn<cr><lf>, where n.. is the pressure value.
US tech support helped me setup DEWESoft to read the same instrument configured as RS-232 (mode is selectable via dip switches).
The Serial Com plug-in settings are:
Request Prefix: \#*?\CR
Response Part 1: Set to "Ignore Response": Stop String: 0\SPC\SPC\SPC
Response Part 2: Set to "Numeric". Stop String: \CR\LF
This method worked to read a single RS-232 device. Note that a wildcard * was sent as the address. I haven't tried it yet, but I am hoping to use the same method with RS-485 multi-drop and using the actual node addresses and hopefully sequence the multiple requests and responses so it is not jumbled.
Matt
Funny I am implementing Mensor CPT pressure transducers as well and read through there manual this morning. I don't see why it would not work with dewesoft.