iX Talk

Say I have a modbus master controller in my t7a. Say I have tag T1 bound to modbus address 40001 and T2 bound to modbus address 40002, and I have valueChange handers on both tags.

For a given polling event, assuming the values of T1 and T2 both changed, can I assume the T1.valueChange will fire before T2.valueChange, because of the order of modbus addresses?

Say I also have T17 bound to modbus address 40017. (I gather from the documentation that the modbus master will fetch T17 in a separate package from T1 and T2). Can I assume the valueChange event for T17 will fire after the valueChange for T2?

Is there some sort of "poll complete" event I can handle, so I could be sure all the tags have values based on the polling event?

You can not assume that the ValueChange event will fire in a given order. iX is multi-threaded and the events are managed by the underlying framework. They may fire in any order.

You can force iX to read a tag in script by calling its Read function, "Globals.Tags.MyTag.Read()".

There is no way to test for a poll complete or trigger a group of tags to be polled.

Oh my. The thing is, my modbus slave reports 32-bit floating point numbers in pairs of registers. For example, modbus address 40001 holds the high 16 bits and 40002 holds the low 16 bits of a floating-point number. Say I have tag T1 on 40001 and T2 on 40002. If I handle valueChanged on T1, I don't know if T2 has the new value or the old value.

Can you think of any solution?

I'll give my solution.

Turns out there is one register in the slave device which always changes. It is a timestamp with one-second resolution. I'll put a valueChange event handler on the tag for that register. When the timestamp changes, I'll set a timer for an absurdly long time - in my case one second, since my polling interval is five seconds. At the end of the one second I am very confident that all the tag values will have settled.

What format does your device store this 32bit float? If it is a standard IEEE (IEEE 754) or ISO International Standard format (ISO/IEC/IEEE 60559:2011) could you not simply choose a single Tag mapped to 40001 and set the controller type to Float? I have been sending and receiving float values (32bit) over the modbus driver for a while now with no problems.

I have had to change settings on some slaves to get the byte order and word swap settings to match the master. One project communicating with a Mettler Toledo Weigh terminal (IND131) required this approach.

Well I'll try that, thanks!