Specifications

78
Schneider Electric
The Modbus Protocol (continued)
"Physical" example of a frame:
b "Master" frames are transmitted by a specific software program.
b "Slave" frames are transmitted by a module of the Digipact range.
b 2-wire cabling with load and polarisation is used for the RS485.
b Measurements are made with an oscilloscope across terminals 8/9 (hot point on
the probe) and terminals 4/5 (cold point) on the module’s 9-pin subD.
The Modbus function 11 "read event counter" is used. All modules have this function
(see description of functions).
b Query.
b Response.
b Note: since transmission speed is 19200 baud:
v 1 bit = 52
µ s,
v 1 byte (1start + 8bits + 1stop) = 520 µ s,
v thus the theoretical length of the response = 4.16 ms (4.14 ms measured).
b Form of the signal measured (start of frame):
Thus we have the address of the slave (02 hex) and the function code (0B hex).
E52397E52398E52400
02 0B 41 17
02 0B 00 00
counter contents
(00 00 in this
case)”.
A4 38
1st byte
2nd byte
+ 5V
OV
- 5V
0
0
00
0
00
0
101
1
0
1
00
0
000
1
1
Start
Stop
Stop
Start
D1
D2 D3
D4
D5 D6
D7
D0
D1
D2 D3
D4
D5 D6
D7
D0
2
0
B
0
=
2.86 ms
Picks up
ine
Guide to Integration
Modbus / Jbus