D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual download pdf (Page 8)

Modbus for Field Technicians

Page 8

2. THERE ARE (WERE) A MAX OF 9999 POINTS OF EACH

DATA TYPE

When Modbus was invented they thought 9,999 items of each memory type

were enough.

Most vendors ignore this limit today – they make clients that can read more

and they make devices which can serve more if required.

Older clients cannot poll for more than 9,999 items.

Even though 9,999 was an arbitrary choice there is a practical limit imposed

by the protocol. The Modbus message uses a 16 bit word to identify the point

number to be read/written. The largest number that can fit in 16 bits is 65535

and hence the highest point number that can be read is point 65535. Most

vendors, these days, allow their software to read any points in this range.

400001, 400002 … 409999….. We call this five digit addressing.

So now we come to a naming problem.

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D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual Page 8