Toshiba T300MVi User Manual download pdf (Page 53)

• Because the transaction is handled locally within the interface, write data

checking is not available. For example, if a write is performed to a register

with a data value that is out-of-range of the corresponding “source port”

object, no Modbus exception will be immediately returned. However, the

point will always reflect the “source port” status and object value. In other

words, if such an out-of-range write attempt is performed, the unsuccessful

“source port” network write can be observed by reading the current

(unchanged) value of the point during a subsequent Modbus transaction.

15.1.1 Coil Mappings

The Modbus slave implementations provide read/write support for coils (0X

references). Accessing coils does not reference any new physical data: coils

are simply indexes into various bits of Modbus holding registers. What this

means is that when a coil is accessed, that coil is resolved by the interface into

a specific holding register, and a specific bit within that holding register. The

pattern of coil-to-register/bit relationships can be described as follows:

Coils 1...16 map to holding register #1, bit0...bit15 (bit0=LSB, bit15=MSB)

Coils 17...32 map to holding register #2, bit0...bit15, and so on.

Arithmetically, the coil-to-register/bit relationship can be described as follows:

For any given coil, the holding register in which that coil resides can be

determined by:













15coil

registerholding

…Equation 1

Where the bracket symbols “ ” indicate the “floor” function, which means that

any fractional result (or “remainder”) is to be discarded, with only the integer

value being retained.

Also, for any given coil, the targeted bit in the holding register in which that coil

resides can be determined by:

16%)1(

−

coilbit …Equation 2

Where “coil” ∈[1…65535], “bit” ∈[0…15], and “%” is the modulus operator,

which means that any fractional result (or “remainder”) is to be retained, with

the integer value being discarded (i.e. it is the opposite of the “floor” function).

From these equations, it can be seen that the largest holding register number

that can be accessed via this coil-to-register mapping method is 4096 (which

contains coil 65535).

For clarity, let’s use Equation 1 and Equation 2 in a calculation example. Say,

for instance, that we are going to read coil #34. Using Equation 1, we can

determine that coil #34 resides in holding register #3, as 3.0625 = 3 r1 = 3.

1 2 ... 48 49 50 51 52 53 54 55 56 57 58 ... 68 69

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Toshiba T300MVi User Manual Page 53

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