Instruction manual

MODBUS System Set Up

32 025-9209

There is one digital input counter for each input of the Model 1708 or Model 1716 RTU.

Each counter consists of two values: the count and the time period since the counter was last

cleared.

The count value increments each time the input activates. When the input is set up as status

mode, the count increments every time the input changes state. If the input is set up as

normally open or normally closed, the count increments when the input is grounded or

disconnected respectively.

The accumulators and counters are accessed through MODBUS holding registers. Each

accumulator or counter value (except for the accumulator sample rate) is spread across three

separate holding registers. The lower four decimal digits are in the first register, the middle

four decimal digits in the middle register, and the upper four decimal digits in the third

2,156,349,078, number of samples of 634,127,890, and sample rate of 2,354:

Table 16. How Accumulator 1 Stores Data

41011 9078 Accumulated Value lower four decimal digits

41012 5634 Accumulated Value middle four decimal digits

41013 0021 Accumulated Value upper four decimal digits

41014 7890 Number of Samples lower four decimal digits

41015 3412 Number of Samples middle four decimal digits

41016 0006 Number of Samples upper four decimal digits

41017 2354 Sample rate (the sample rate uses one register)

Some MODBUS software packages (e.g. WonderWare’s Intouch) can automatically combine

these registers into a single value. Others, (e.g. National Instrument’s Lookout) require the

user to combine the registers into a single value in a “script”. For example, in Lookout, the

“script” for the accumulated value might look like the following equation,

()

Rtu1.41013 10000 Rtu1.41012 10000 Rtu1.41011×+ ×+

Queries to read an accumulator or counter must request all associated registers or an illegal

data address exception will be returned. An illegal data address exception will also be

returned to queries that overlap accumulator or counter address boundaries by requesting

more than nine registers.

Writing to the first holding register address used by particular accumulator or counter will

cause it to be cleared. The value written does not matter; the act of writing to the register

causes the clearing. It is not possible to clear just part of an accumulator or counter. For

example, you cannot clear the accumulator total without also clearing the number of samples.