8 - 4 VM600 networking manual MAVM600-NET/E
Edition 9 - February 2018
Description of Modbus RTU protocol
SETTING UP A MODBUS CONNECTION (CPUM SW VERSION 067 OR EARLIER)
The connected equipment (VM600 rack) detects the start of a message when any valid
character (containing either its address or the address 00) has been received after a silent
interval of at least 3.5 CT. The end of a message is interpreted by a silent interval of the same
duration.
8.4.2 Communications parameters for the VM600
By default, the communications parameters for the VM600 are:
• Baud rate 9600
•Parity Even
• Number of bits 8
• Number of stop bits 1
• Slave address 1.
These settings can be modified if required by editing the appropriate configuration file stored
on the CPUM (for example, “mbcfg.2”, “mbcfg.3”, “mbcfg.4” or “mbcfg.5”). This is done using
either a VT100 terminal or a terminal emulator in the Windows environment).
See 2.3 Modbus configuration files for additional information.
8.4.3 Modbus functions supported by the VM600
The Modbus functions supported by the VM600 are shown in Table 8-2.
Request Frame
Figure 8-3: Timing considerations
Request Frame
Response Frame
Addr. CRC Addr. CRC Addr. CRC
T
1
T
2
T
3
T
1
= time between two characters (min. = 0, max. = 3.5 CT)
T
2
= time between request and response (min. 3.5 CT)
T
3
= time between response and next request (min. 3.5 CT)
CT = 1 character time (= the time needed to transmit one
character consisting of 10 bits)
Baud
(bps)
Time for
3.5 CT
1200 30 ms
2400 15 ms
4800 8 ms
9600 4 ms
19200 2 ms
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