20
devices. To solve this problem and allow integration with all Modbus masters,
the ASD-NANOCOM interface implements “register remapping”.
Inspection of the Toshiba serial communications manual reveals that the
drive’s parameters are divided into two distinct regions within the full 0x0000 ~
0xFFFF ASD register map: the “lower address” range covers from 0x0000 to
0x09FF, and the “upper address” range covers from 0xFA00 to 0xFFFF (not all
values within those ranges access valid ASD registers, of course). While all of
the parameters residing in the “lower address” range are directly accessible by
“register limited” masters, none of the parameters residing in the “upper
address” range are. Via register remapping, the ASD-NANOCOM interface
automatically maps all “upper address” ASD parameters into the contiguous
region immediately following the “lower address” range. To take advantage of
this feature, simply drop the initial “F” on any parameter index in the “upper
address” range and the resultant Modbus register request will automatically
access the correct internal ASD parameter.
For example, if we again wish to write a frequency command to the drive
(register FA05), we would change the FA05
16
to 0A05
16
and then convert this
index to decimal to obtain the resultant Modbus register (2565
10
). Therefore,
writing to holding register 2565 will issue a new frequency command to the
drive. Of course, it is also possible to access the “upper address” parameters
at their original locations, so the RS-485 frequency command will be accessed
the same whether writing to holding register 2565
10
or 64005
10
.
8.6
Coil Mappings
The ASD-NANOCOM interface provides read/write support for Modbus coils.
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:
+
=
16
15
coil
register
holding
…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.
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