905U-G Wireless Gateway
User Manual
Page 125
©September 2004
configuration is the recommended option if use of the GDB Offset and Count parameters is
required (see section 4.11.4).
Two sets of six switches are available: Node Address (S1, the left-most set of switches,
closest to the D-SUB connector), and Source Address (S2, the right-most set of switches).
Address settings for both switches use the same binary format illustrated in the table below.
1
MSB
2
3
4
5
6
LSB
Function
ON
ON
ON
ON
ON
ON
Node Address set to 1
ON
ON
ON
ON
ON
OFF
Node Address set to 2
ON
ON
ON
ON
OFF
ON
Node Address set to 3
---
---
---
---
---
---
OFF
OFF
OFF
OFF
OFF
ON
Node Address set to 63
OFF
OFF
OFF
OFF
OFF
OFF
Node Address set to 64
4.13.3
Protocol & Supported Functions
Devices on a Modbus Plus network have two ways of exchanging data. One is through fast
cyclic I/O data called Global Data, and one through a somewhat slower Modbus protocol for
point-to-point parameter data transfer. The 905G supports both Global Data and point-to-
point data, however the module cannot initiate point-to-point commands but only respond to
and accept point-to-point commands initiated by other nodes on the network.
Modbus Plus is a token bus network. This means that each device on the network will receive
the token on a cyclic basis. When a device on the network receives the token it is able to
broadcast up to 32 words of Global Data. All other devices on the network will ‘see’ this
data, and depending on their configuration have the option to use some, or all, of the
broadcast data. Consequently, the 905G Modbus Plus module supports up to 32 words of
Global Outputs (i.e. Data To Network) and up to 32 words of Global Inputs (i.e. Data From
Network).
The 905G also supports point-to-point data, however the module cannot initiate point-to-
point commands but only respond to and accept point-to-point commands. The 905G Modbus
Plus supports only the following point-to-point operations on Modbus 40000 (4X) registers:
-
(0x03) Read holding Registers
-
(0x06) Preset Single Register
-
(0x10) Preset multiple Registers
The 905G Modbus Plus supports the following exception responses:
-
(0x01) Illegal function for the addressed slave
-
(0x02) Illegal data address within the information field for the addressed slave
-
(0x03) Illegal data value in the information field for the addressed slave
The 905G Modbus Plus supports up to 1024 words of output data and 1024 words of input
data. Converting this to 40000 registers, the possible output registers (Data To Network)
