Message Timing; Modbus Functions; Supported Modbus Functions; Read Actual Values Or Setpoints - GE Multilin 745 Communications Manual

MODBUS PROTOCOL

Message timing

Supported Modbus
functions
Read actual values or
setpoints
2–4
Communication message synchronization is maintained by timing constraints. The
receiving device must measure the time between the reception of characters. If three and
one half character times elapse without a new character or completion of the message,
then the communication link must be reset (i.e. all slaves start listening for a new query
message from the master). Thus at 1200 baud a delay of greater than 3.5 x 1/1200 x 10 =
29.2 ms will cause the communication link to be reset. At 9600 baud a delay of greater
than 3.5 x 1/9600 x 10 = 3.6 ms will cause the communication link to be reset. Most master
query messages will be responded to in less than 50 ms.

Modbus functions

The second byte of every message is the function code. Modbus defines function codes of
01h to 7Fh. The GE Multilin SR-series Modbus protocol supports some of these functions, as
summarized below.
Table 2–1: GE Multilin Modbus function codes
Function Code Definition
Hex Dec.
03 3 Read actual values
or setpoints
04 4
05 5 Execute operation
06 6 Store single setpoint Write a specific value into a single
10 16 Store multiple
setpoints
Since some programmable logic controllers only support function codes 03h (or 04h) and
10h, most of the above Modbus commands can be performed by reading from or writing
to special addresses in the 745 memory map using these function codes. See Function
code substitutions on page 2–11 for details.
Modbus implementation: Read holding registers
GE Multilin implementation: Read actual values or setpoints
Since some PLC Modbus implementations only support one of function codes 03h and 04h,
the 745 interpretation allows either code to be used for reading one or more consecutive
setpoints or actual values. The data starting address determines the data type being read.
Function codes 03h and 04h are therefore identical.
The GE Multilin implementation of Modbus views "holding registers" as any setpoint or
actual value register in the 745 memory map. Registers are 16 bit (two byte) values
transmitted high order byte first. Thus all GE Multilin setpoints and actual values in the
memory map are sent as two byte registers. This function code allows the master to read
one or more consecutive setpoints or actual values from the addressed slave device.
The maximum number of values that can be read in a single message is 120.
Description
Read actual value or setpoint registers
from one or more consecutive memory
map registers
Perform 745 specific operations
setpoint register
Write specific values into one or more
consecutive setpoint registers
745 TRANSFORMER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
CHAPTER 2: MODBUS PROTOCOL
Substitute
04h
03h
10h
10h
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