Sending %M bits using ModbusTCP - RX3i

For Modbus TCP in the Rx3i – it’s best to think of the Modbus Standard Map to line up like below to Ref Memory

(NOTE: only Reference memory can be polled via Modbus)

Traditional Modbus Address                                               Rx3i Ref Address

0xxxx – Coils                                Map to                %Qxxxx – traditionally Discrete Outs

1xxxx – Discrete Input Table        Map to                %Ixxxx – traditionally Discrete Inputs

3xxxx – Input Registers                Map to                %AIxxxx – traditionally Analog Inputs

4xxxx – Holding Registers            Map to                %Rxxxx  – traditionally internal Registers

Generally, a best practice is to “reserve” some reference area for Modbus polls then – MOVE the Bits and Words they want available via Modbus to a %Q, %I, %AI, %R – a suggested best practice. (this is usually only helpful if you know upfront Modbus will be a requirement)

HISTORY *** %T vs %M – Yes, these are both internal Bits. 

In the Series 90-30 – you could not designate a Bit to be retentive or Non-Retentive, so you would us a %M (retentive internal bit) or a %T (temporary) – non-retentive internal bit. 

In PACSystems – you can designate any Bit to be retentive, so this distinction no longer makes sense, but we kept the same memory design so that Serie 9030 to Rx3i upgrades would be easy and compatible. 

If you need to share %M or %T – I suggest you move them to a %Q or %I not being used in the hardware and then poll them. 

SYMBOLIC – Symbolic memory by definition is not mapped to Ref memory – there is a function in PACSystems - MOVE_TO_FLAT – this will take Symbolic memory and move it to Reference memory. Useful if you have to “add in” Modbus TCP after the fact. (There is also a MOVE FROM FLAT)