Perform Modbus RTU Command
rt, txt = mbo:cmd(adr, fnc, [tfmt, ttab, [rfmt, ms]] )
rt, txt = modbus.cmd(cpo, adr, fnc, [tfmt, ttab, [rfmt, ms]] )
Send a command with Modbus RTU and get the response.
| Parameter | Type | Description |
|---|---|---|
mbo |
object table | Modbus Object |
cpo |
object | COM Port Object |
adr |
integer | Slave address |
fnc |
integer | Function Number |
tfmt |
string | Transmit data format string ([Modbus Format Strings]) |
ttab |
table | Transmit data table |
rfmt |
string | Receive data format string ([Modbus Format Strings]) |
ms |
integer | Receive timeout |
| Returns | Type | Description |
|---|---|---|
rt |
table | Values (as specified by rfmt) |
txt |
string | Any remaining data |
The indexed tables ttab is used, along with tfmt, to create a command string that is passed to modbus.tx.
The reply formatting is optional.
For example, to write to register 1000:
modbus.cmd(cpo, 1, 16, 'WWBW', {1000, 1, 2, 7878})
-- Register 1000 will have 2 bytes written as 7878
The modbus.cmd method will then call modbus.rx, and any reply is pulled apart using the rfmt specifiers to create a table of results.
For example, the Read Holding Registers command (0x03) function will return a byte-count, followed by n x 16-bit values (rfmt = 'BWWW' for 3 values):
local t = modbus.cmd(cpo, 1, 3, 'WW', {2, 3}, 'BWWW')
if t
then
-- t[1] = Number of bytes
-- t[2] = register 0002
-- t[3] = register 0003
-- t[4] = register 0004
_, reg1, reg2, reg3 = unpack(t)
end
Note the use of Lua's unpack(t) to turn a table into a list of separate values.
Use modbus.rhr as a complete solution for Read Holding Registers
The above example is purely for education.