GE b30 Communications Guide

2-4 UR FAMILY – COMMUNICATIONS GUIDE
MODBUS FUNCTION CODES CHAPTER 2: MODBUS COMMUNICATION
2
2.2 Modbus function codes
2.2.1 Supported function codes
Modbus officially defines function codes from 1 to 127 though only a small subset is generally needed. The relay supports
some of these functions, as summarized in the following table. Subsequent sections describe each function code.
Table 2-4: Function codes
2.2.2 Read actual values or settings (function code 03/04h)
This function code allows the master to read one or more consecutive data registers (actual values or settings) from a
relay. Data registers are always 16-bit (two-byte) values transmitted with high order byte first. The maximum number of
registers that can be read in a single packet is 125. See the Reading PMU records
section on page 2-11 for the data
registers.
Since some PLC implementations of Modbus only support one of function codes 03h and 04h. The UR interpretation allows
either function code to be used for reading one or more consecutive data registers. The data starting address determines
the type of data being read. Function codes 03h and 04h are therefore identical.
The following table shows the format of the master and slave packets in Modbus RTU. Modbus TCP/IP ADUs have a MBAP
instead of slave address, and CRC is in another stack layer. The example shows a master device requesting three register
values starting at address 4050h from slave device 11h (17 decimal); the slave device responds with the values 40, 300,
and 0 from registers 4050h, 4051h, and 4052h, respectively.
Table 2-5: Master and slave device packet transmission example
Algorithm 1. FFFF (hex) --> A
2. 0 --> i
3. 0 --> j
4. Di (+) Alow --> Alow
5. j + 1 --> j
6. shr (A)
7. Is there a carry? No: go to 8; Yes: G (+) A --> A and continue.
8. Is j = 8? No: go to 5; Yes: continue
9. i + 1 --> i
10. Is i = N? No: go to 3; Yes: continue
11. A --> CRC
Function code Modbus definition GE Multilin definition
Hex Dec
03 3 Read holding registers Read actual values or settings
04 4 Read holding registers Read actual values or settings
05 5 Force single coil Execute operation
06 6 Preset single register Store single setting
10 16 Preset multiple registers Store multiple settings
Master transmission Slave response
Packet format Example (Hex) Packet format Example (Hex)
SLAVE ADDRESS 11 SLAVE ADDRESS 11
FUNCTION CODE 04 FUNCTION CODE 04
DATA STARTING ADDRESS - high 40 BYTE COUNT 06
DATA STARTING ADDRESS - low 50 DATA #1 - high 00

GE b30 Specifications

General

Brand	GE
Model	b30
Category	Relays
Language	English