12.0
COMMUNICATION (OPTIONAL)
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MAGPOWR Spyder-Plus-S1 Tension Control
MI 850A351 1 C
Page 61 of 68
12.6 MODBUS/TCP COMMUNICATION
The Spyder acts as a Modbus/TCP slave device. It supports the function codes Read Holding Registers (code 03),
Read Input Registers (code 04), Write Single Register (code 06), and Write Multiple Registers (code 16). Either
Read Holding Registers or Read Input Registers may be used to read data, there is no difference between them.
The Spyder data size on Modbus is 228 Modbus registers of input and output starting at Modbus register
address 1. The table in Section 12.7 shows how all the Spyder parameters are mapped onto Modbus registers.
There are three parameters in the Network menu to configure the Modbus communications, Modbus Address,
Modbus Byte Order, and Modbus Word Order. Modbus Address specifies the Modbus address of this slave device
and ranges from 0 to 255.
The Spyder contains data which does not fit into the standard Modbus register size of 16-bits (2 bytes). Some
Spyder data consists of multiple consecutive Modbus registers. The following table shows the correspondence
between the Spyder data types and the number of Modbus registers used to represent the data.
Spyder Data
Number Modbus Registers
Comments
BYTE
8-bit Unsigned Byte
1
High byte forced to 0
UINT16
16-bit Unsigned Integer
1
UINT32
32-bit Unsigned Integer
2
FLOAT
4-byte Floating Point
2
STRING
32-byte Strings
16
Since some data values are contained in multiple Modbus registers the function code 16, Write Multiple Registers
must be used to write these data values. If a write occurs to only one of the Modbus registers corresponding to a
two-word parameter then the Spyder will ignore the write. If a write to a 16 word string does not write all 16
registers then the Spyder will ignore the write. This prevents the Spyder from receiving only part of a data value.
This could be problematic if Setpoint Tension or other control parameter was corrupted by only writing a portion of
the new data value. Reads do not have this restriction.
The Modbus standard specifies data as being transmitted Most Significant Byte first. To support master devices
which transmit data Least Significant Byte first the Spyder allows switching between the two. For multiple register
data there is a configuration parameter to specify whether words (registers) are transmitted Most Significant Word
first or last. These two configuration parameters are in the Network menu. The default is Most Significant Byte first
and Most Significant Word first. Note: Strings are always transmitted left most character first.
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