Simpson Electric G100, G200, G300, G400 Communications Guide
GIMA
Modbus Communications
December 2008
Safety & Programming
GIMA
1 Safety
The GIMA is intended for connection to dangerous voltages giving a risk of electric shock.
Refer to the safety/installation instructions in the GIMA Operating Manual before connecting
the communications.
WARNING
The meter contains no user serviceable parts. Installation and commissioning should only be
carried out by qualified personnel
2 Programming
Meters fitted with the Modbus option have two additional stages in the front panel programming
menu.
To enter programming mode:
Hold Amp and scroll together for 5 Seconds.
Press repeatedly until the required setup page is displayed.
RS485 Baud Rate
Modbus Meter ID
9
bAU
R
A
TE
6
C
Ad
U
b
d
d
0
E
r
0
Press p or qto select from the
list of valid baud rates. (4800-
19200)
Press
Press p or q to increment or
decrement the value within the
valid range (1-247).
Press to accept the set value.
to accept the set value.
For full information on entering and using programming mode refer to the latest “GIMA
Operators Manual "
1
Page 2 GIMA
Connection GIMA
3 Connection
3.1 Cable Selection
A dedicated, screened twisted pair cable is required to provide basic RS485 connection. A
second twisted pair may be used for 0V connection if required. The cable should be chosen to
suit the data rate and maximum length to be installed. The EIA RS-485-A standard provides
curves that relate cable length to data rate for 24 AWG screened, twisted pair, telephone cable
with a shunt capacitance of 50pf/m. For baud rates up to 19,200 the standard suggests a
maximum length of 1200m for this type of cable. If other types of cable are to be used it is
recommended that the cable supplier is consulted as to the suitability for use with RS485 to
19,200 baud.
3.2 Signal 0V and Cable Shield
A signal 0V termination is provided on each meter. Although RS485 does not strictly require a
signal 0V, it is recommended this is connected as shown in the diagram below. This creates a
known reference for the isolated RS485 system thereby reducing potential common-mode errors
in the meter’s RS485 driver circuit.
A cable shield is used to attenuate noise picked up from external sources. This should be
continuous, and cover as much of the signal pairs as possible. It is recommended that the shield
should be connected to ground at the host (PC) only. The cable shield should not be used as the
0V connection.
3.3 Terminating Resistors
In order to minimise signal errors due to noise over long cable lengths, terminating resistors may
be fitted. These match the RS485 device impedance to that of the cable. Two 120Ω resistors,
one at the host port terminals and the other at the most remote meter terminals are recommended
for this purpose.
3.3.1 Connection To Meters
The bus should be taken to meters at each location for termination, using the meter terminals as a
loop in-out connection. The use of spurs should be avoided wherever possible.
Figure 3-1 Basic RS485 To Internal Modbus Option
Note 1 Depending on application these leads
may need to be reversed.
Page 3 GIMA
Connection GIMA
4 Protocol
4.1 Modbus Commands
The GIMA meter supports the following standard Modbus commands:
Command Function Broadcast
03 Read Multiple Holding Registers No
04 Read Multiple Input Registers No
06 Preset a Single Register Yes
08 (SF=00) Sub Function 00 only (Loop Back) No
16 Preset Multiple Registers Yes
4.2 Exception Responses
If the meter receives a Modbus command, with no errors and a valid address, it will attempt to
handle the query and provide an appropriate response. If the meter cannot handle the query a
standard Modbus exception response is sent (except broadcast queries). An exception response
is characterised by its function byte which has 80H added to that sent in the query. The following
exceptions codes are supported:
Code Function
1 Preset data is out of range for parameter
2 Function cannot access requested register address
Page 4 GIMA
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