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lua-libmodbus

Lua bindings to libmodbus

Parameters are mostly as per libmodbus documentation, but values are returned directly, in tables, rather than in pointers and return codes. Instead of a return code, you would get a lua style "nil, error_msg" return pair.

Of particular relevance to modbus, is that tables are addressed with lua style 1 based counting, but converted to zero based internally. ie:

	res = dev:read_registers(0x2000, 2)
	print(#res) -- prints 2
	print(res[1]) - prints register address 0x2000, _not_ 0x2001

API documentation (Generated from last release)

Status

  • Client bindings for RTU/TCP and almost all operations.
  • Some helpers for working with 16/32bit signed/unsigned and floats in multiple registers (API is not necessarily nailed down, comments welcome)
  • Server side limited to receive and reply exception. Needs thoughts on how to handle the mapping objects.
  • Compatible with both 3.0.x and 3.1.x but you must run with the version you compiled with.

Compile

You need Lua and libmodbus development packages (headers and libs) to build lua-libmodbus.

Compile with

make

You can override the pkg-config package name to set a specific Lua version. For example:

make LUAPKG=lua-5.1

Example usage

See demo.lua and other sample files.

Here is a simple example printing out 10 registers at 0x2000, then writing 5 registers back to that address

mb = require("libmodbus")
local dev = mb.new_tcp_pi("192.168.255.74", 1502)
local base_address = 0x2000
dev:connect()
local regs, err = dev:read_registers(base_address, 10)
if not regs then error("read failed: " .. err) end
for r,v in ipairs(regs) do
        print(string.format("register (offset %d) %d: %d (%#x): %#x (%d)",
                r, r, r + base_address - 1, r + base_address -1, v, v))
end

Another example writing registers instead

mb = require("libmodbus")
local dev = mb.new_tcp_pi("192.168.255.74", 1502)
dev:connect()

-- will autoround 32.98 to 32.  no magic support for writing floats yet :|
-- numbers are cast to uint16_t internally.
local res, err = dev:write_registers(0x2000, { 0xabcd, 32.98, 0xfffe, 0xabcd, -1 })
if not res then print(err) end

Testing

Some spec files for use with busted are provided. Some of them expect to have real hardware to talk to, so you may wish to disable them.

busted --exclude-tags=real