8 SERIES PROTECTIVE RELAY PLATFORM – COMMUNICATIONS GUIDE
12-1
8 Series Protective Relay Platform
Chapter 12: About the Modbus
Memory Map
GE
Grid Solutions
About the Modbus Memory Map
Accessing Data via the User Map
The 8 Series relay has a powerful feature, called the User Map, which allows a computer to
read up to 125 non-consecutive data registers (setpoints or actual values) by using one
Modbus packet. It is often necessary for a master computer to continuously poll various
values in each of the connected slave relays. If these values are scattered throughout the
memory map, reading them would require numerous transmissions and would burden the
communication link. The User Map can be programmed to join any memory map address
to one in the block of consecutive User Map locations, so that they can be accessed by
reading these consecutive locations.
The 8 Series relay contains a user-definable area in the Modbus memory map. This area
allows re-mapping of the addresses of any Actual Values registers. The User Definable
area has two sections.
1.
A Register Index area (40524 to 40648) that contains 125 Actual Values register
addresses
2.
A Register area (30524 to 30648) that contains the data at the addresses in the
Register Index
Register data that is separated in the rest of the memory map may be re-mapped to
adjacent register addresses in the user-definable registers area. This is accomplished by
writing to register addresses in the user-definable register index area. This allows for
improved throughput of data and can eliminate the need for multiple read command
sequences. The user-definable register index is stored as a setpoint and therefore it is
“remembered” even when the power is removed.
Default User Map
Common actual values have been defaulted for the user. Below are the default registers.
Table 12-1: Default User Map
User Map Address
Description
Modbus Address
User Map Address 1,2
Device Status
30712
30713
User Map Address 3,4
LED Status
30708
30709
User Map Address 5,6
Contact Input 1-32
30700
30701
User Map Address 7,8
Contact Output 1-32
30706
30707
