How do I send Hex data?

Question

I am trying to communicate with a modbus slave via either modbusTCP or modbus serial. The manuf. (partlow) has an ASCII communications manual (http://www.partlow.com/uploadedFiles/Downloads/1160%20ASCII%20Comms%20Manual.pdf) which, looks like it differs from the standard communication methods (http://en.wikipedia.org/wiki/Modbus). A lot of existing code out there is setup to work with normal modbus addressing of coils and such, where it seems (at least to me) to be different with these guys.

So, via ruby or perl, how can I send hex data? I may be doing everything fine, but, if I write "0DFA" to a serial port... is that ok? or do I need to convert it into a lower layer first, or denote it somehow?

Been working on this a lot and may have myself mixed up (making things out to be more complicated than they are) but, i am trying to establish comm with this meter, and I can see the TX activity light blink but no RX, which means my data format is wrong...

Been working off this mostly (and a few perl snippets here and there, trying to find something that works): http://www.messen-und-deuten.de/modbus.html

I am communicating through a terminal server, which accepts modbusTCP (which this script uses) but i'm having trouble applying whats in the comm manual to the code above, to get the packet formatted correctly.

Answer 1

Are you talking about raw data? There are several ways, including

print HANDLE "\x{OD}\x{FA}";

printf HANDLE "%c%c", 0x0D, 0xFA;

print HANDLE "\015\372";       # octal notation

print HANDLE pack("C*", 0x0D, 0xFA);

syswrite HANDLE, "\x{OD}\x{FA}", 2;

Answer 2

I would recommend you look at the RModBus library to help handle some of the intricacies of packet formation over TCP/IP from inside the Ruby language.

It is always possible that the device you are communicating with requires, or conversely avoids the modicon notation. That was a bit of a hiccup when I first tried reading registers from a PLC. The other "gotcha" that I've found with Modbus is that some of the addressing systems are offset by one due to quirkiness in their implementation.