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Step 3: Specify Master Memory Area
The third instruction in the RX or WX sequence is
a Load Address (LDA) instruction. Its purpose is to
load the starting address of the memory area to be
transferred. Entered as an octal number, the LDA
instruction converts it to hex and places the result
in the accumulator.
For a WX instruction, the DL250-1/260 CPU
sends the number of bytes previously specified
from its memory area beginning at the LDA
address specified.
For an RX instruction, the DL250-1/260 CPU
reads the number of bytes previously specified
from the slave, placing the received data into its
memory area beginning at the LDA address
specified.
NOTE: Since V-memory words are always 16 bits, you may not always use the whole word. For example, if
you only specify 3 bytes and you are reading Y outputs from the slave, you will only get 24 bits of data. In
this case, only the 8 least significant bits of the last word location will be modified. The remaining 8 bits
are not affected.
Step 4: Specify Slave Memory Area
The last instruction in our sequence is the WX or
RX instruction itself. Use WX to write to the slave,
and RX to read from the slave. All four of our
instructions are shown to the right. In the last
instruction, you must specify the starting address
and a valid data type for the slave.
• Direct NET slaves – specify the same address in the
WX and RX instruction as the slave’s native I/O
address
• Modbus DL405 or DL205 slaves – specify the same
address in the WX and RX instruction as the slave’s
native I/O address
• Modbus 305 slaves – use the following table to
convert DL305 addresses to Modbus addresses
DL205 User Manual, 4th Edition, Rev. B
4–43
Chapter 4: System Design and Configuration
1
2
3
4
5
6
7
8
9
10
11
12
13
14
A
B
C
D
6 0 0 0
(octal)
LDA
O40600
4
Starting address of
master transfer area
V40600
MSB LSB
0 15
V40601
MSB LSB
0 15
LD
KF101
LD
K1 28
LD A
O4 0 6 00
RX
SP 11 6
Y 0
DL305 Series CPU Memory Type–to–Modbus Cross Reference
PLC Memory Type
PLC Base
Address
Modbus
Base Address
PLC Memory
Type
PLC Base
Address
Modbus
Base Address
TMR/CNT Current Values R600 V0
TMR/CNT
Status Bits
CT600 GY600
I/O Points
IO 000 GY0 Control Relays CR160 GY160
Data Registers
R401,R400 V100 Shift Registers SR400 GY400
Stage Status Bits (D3-330P only)
S0 GY200