Subject to change without notification.
This
document responds to the frequently asked questions concerning the Engiby
Modbus Driver (versions 1.xx).
Attention : The Modbus 1 library is acually obsolete
and replaced by a Modbus 2 package.
For more info, go to www.engiby.ch/modbus/tcpfaq.htm and www.engiby.ch/modbus/modcmp.htm.
This page is still
available for existing application using Modbus 1.
Further
useful information about Modbus
If you are
looking for more information about the various Modbus protocol visit the Modbus
organization sites at http://www.modbus.org and http://www.modbus-ida.org.
When you
will build up your first Modbus installation, you may whish to test the Modbus
driver in the PCD first. You can find very useful test software, called
ModScan32, for little money by Wintech at http://www.win-tech.com. Further tools
are available from the Modbus sites indexed above.
PG4
and PG5 licenses
PG3, PG4
version 1.4 and PG4 version 1.0.80:
These tools are not anymore supported with the versions 1.5x, and the
following. IL programs from PG3 and PG4 are not anymore supported with these
versions. We recommend updating all old tools to PG5 1.4. Contact Engiby if one
of these old programming tools must be used.
Modbus
drivers 1.4x:
These versions was designed and licensed for the PG4 Tool and can be used with
the version 2.0.2xx of PG4. You need an update to use the Modbus driver on PG5.
Modbus
driver 1.50 and 1.51:
These versions were an intermediate step for PG5 versions 1.0.xxx. You should
not use it anymore and update it to the latest 1.7x version.
Modbus
driver from 1.52 on:
These versions can be used in PG4 versions 2.0.2xx as well as PG5 versions
1.0.xxx and 1.1.xxx.
Modbus
driver from 1.60 on:
These versions can be used in PG5 versions 1.0, 1.1, 1.2 and 1.3 but not
anymore on PG4. Once you have updated your PG4 to PG5 and installed the Modbus
driver version 1.6x you can move your Fupla programs
into a PG5 project and continue to use it. Refer to the SAIA's instructions to
import PG4 projects into PG5.
For IL
programs in PG5 and old programs imported from PG3 or PG4, you need to adapt
the source file according to the new examples. You also need a Modbus driver
version 1.55 at least. See below.
Modbus
driver 1.7x:
These versions can be used in PG5 versions 1.2, 1.3, and 1.4.
Modbus
driver 1.8x:
These versions can be used in PG5 versions 2.0. In this case a Modbus 2 package
must first be installed. Modbus 1.8x will use the same license key as Modbus 2.
Trouble
shooting
This document
may help you for trouble shooting of the Modbus 1 Driver: www.engiby.ch/modbus/modtrb.htm.
Last
modifications of the driver
Here
you can find the list of the last modifications of the Modbus driver 1.
Programming
tool and programming languages
S-Fup and
IL programs can be realized. However, the user's manual is mainly based on the
S-Fup Fboxes. Remember that the best comfort is reached when using S-Fup. You
need at least the following Modbus driver versions for IL programs:
|
PG5 1.0
and 1.1 |
don’t
use it anymore |
|
PG5 1.2
and 1.3 |
Modbus
driver version 1.55 |
|
PG5 1.4 |
Modbus
driver version 1.73 |
|
PG5 2.0 |
Modbus
driver version 1.80 |
An
example of IL programs for Master and Slave can be downloaded here
(modbusil.zip). The project is made for PG5 1.2 and can also be used in PG5 1.3 and
1.4. After the download, use the File-Restore command in PG5 to add the example
in your projects.
Protocol
version
The two modes
of Modbus are supported by the Driver: the ASCII and RTU modes. J-Bus is
another name for Modbus and is fully compatible with Modbus ASCII or RTU. Note
that the RTU is two times more efficient as the ASCII. RTU needs a 8 data bits transmission.
Modbus-TCP (or UDP) is supported by the new Modbus 2 Driver.
Modbus plus cannot be supported. This version need special hardware and can
only be connected via a gateway.
Master
/ Slave
The Driver
supports both, Master and Slave functions. However it is important to know if
the PCD will be master or slave. Only the Master can (must) initiate Write and
Read requests. Therefore a slave cannot directly communicate to another device
on the network.
Support
of several line with Modbus driver in the same PCD
The
Master and Slave Driver Fboxes are made to support one serial line only. If
multiple lines are to be used in Master and/or in Slave, a new file must be
created in the same project for each serial line. For the lines in Master mode,
the reception and transmission Fboxes have to be programmed in the file
corresponding to the serial line being used. The fact that multiple lines are
used with the same PCD must be taken into account for evaluating the admissible
load on the CPU. The reaction time will also be longer.
Supported
functions as Master
|
Function
1 |
Receive
Binary |
Check
that the required functions are supported. Contact Engiby if further functions
are desired.
Supported
functions as Slave and interpretation.
|
Function
1 |
Reading
flags |
Check
that the required functions are supported. Contact Engiby if further functions
are desired.
Hardware
connection
RS232
RS422 can be used for point to point (one master, one slave only) connection
without restriction. Remember that the maximum cable length is 15m. Tx and Rx wires must be crossed over. Hardware handshake is
not needed for the driver. The CTS and the DTR outputs are set to High and can
be used if required by the partner device.
RS485
RS485
must be used for multi-drop connections (one master, several slaves). The RS485
bus must be installed respecting the recommendation standing for an S-Bus
network. See the SAIA S-Bus manual. The 2 Tx/Rx+ and Tx/Rx- wires must all be
strain connected (not crossed over). Terminals can also be marked A and B. In
this case B is the Tx/Rx- wire and A is the Tx/Rx+.
External converters (RS232<->RS485) from SAIA cannot be used. Only
converters with automatic switching can be used.
RS422
RS422 can be used for longer point to point connections (one master and one
slave only) without restriction. The cable length can be up to 1000m. In a
multi-drop topology, RS422 can only be used if the PCD is Master. The
Using
the PGU port
The PGU
port is the port 0 on PCD 1, 2, 3 and 4. It is the port 4 on PCD6.M3.
Since version 1.56, the option 'Channel 0 PGU' is available for PCD1, 2, 3 and
4. With this option, the Modbus driver is automatically restarted when the PGU
cable is unplugged.
On PCD3,
you can check the option 'Full RS232 handshaking on Port 0' in the hardware
configuration and use the port 0 without restriction (with the normal option
'Channel 0').
For
former versions and other PCD systems the PGU port can be used with the
following restrictions: the line must not be configured for S-Bus PGU. No PGU
cable must be plugged in when the PCD starts. If a PGU cable is connected, the
driver is disabled and the line is forced in PGU mode. This may cause error at
the Modbus Driver Fbox. To re-enable the Modbus driver, the PCD must be
restarted after disconnecting the PGU cable.
Port
10 as port 3 on PCD3.M5xxx
The PCD3.M5xxx
systems have two D-Sub connectors on the left side. The right side connector is
equipped as RS485 port. It can be configured for Profibus communication as port
10. If not configured, this port can be used for Modbus (or other protocols)
communication but as port
Firmware
version
To
determine if an old CPU will support the Modbus driver, you have to check the
following:
The FW must support the SYSRD and the SYSCMP instructions for the so called
'free running counter'. If you have a Modbus version 1.41 or higher and you
exchange Floating point values, it must also support the SYSWR K 7000
instruction. The table of CPU and firmware related to each instruction is
delivered by SAIA.
Value
format
Basically
the Modbus protocol supports 16 bits values only. The PCD has 32 bits
registers. 16 bits values are converted to 32 bits signed values when received
in the PCD.
Integer
32 bit values
Using
double word combination, 32 bits integer values can be exchanged. This is
supported since version 1.43 of the driver. The position of the higher and the
lower word in the telegrams must be checked and adjusted in the driver Fbox.
Then in the RCV Fboxes select the format 32->32.
Floating
point values
Floating
point values (32 bits) can also be exchanged using double word combination. The
position of the higher and the lower word in the telegrams must be checked and
adjusted in the driver Fbox. You must use the Float Fbox to read floating point
values.
Depending on the manufacturer, various terms are used to specify the word order
for 32 bits values.
Fbox option: Modicon = LSW-MSW = Big Endian = Intel
Fbox option: IEEE = MSW-LSW = Little Endian = Motorola
Addressing
For a PCD
Master, the address of the available values in the Slaves must be known. In the
SEND and RCV Fbox, addresses are given in integer format. Convert Hex format to
integer if necessary.
For a PCD Slave, all resources are available by the Master. Be sure the Master
doesn't access the dynamic allocated resources.
The
standard Modicon notation uses the '4x' and the '3x' reference. The 4x
reference concerns the Holding registers while the 3x reference stands for the
Input registers (Timers and counters in PCD). The addresses are then given in
form 40'001 or 30'123 which means Holding register 1 and Input registers 123.
Don't mix it up with the function code. Reading the 4x reference is made with
function 3 while 3x references are read with the function 4.
The
Modicon implementation supports address ranges starting from 1 (instead of 0).
In this case the offset of one element can be corrected by the driver by
selecting the corresponding option.
Difficulties
can appear with Supervisors using the addressing scheme of Modicon devices with
32 bits values. Modicon uses two 16 bits registers for one 32 bits value.
Therefore each second register address is skipped. The SAIA-PCD has 32 bits
registers and by default, all registers are used (no skip). Since driver
version 1.46, this particular addressing (Skip each 2nd register)
can be set in the Slave driver.
Performances
The
performances of the driver depend on the transmission speed and the reaction
time of the CPU.
The reaction time of the CPU is affected by:
- The number of values
asked or sent in each MODBUS requests: fewer values = faster response.
- Other applications: each
application extends the cycle and the reaction time.
- The other serial
connections: Each active serial line will reduce the performance.
- The protocol version:
The performance is reduced by half with the ASCII protocol.
Load
on the CPU
The
maximum load on the CPU is reached during data reception and depends on the
amount of transferred values. The following measurements have been made with
the Driver version 1.43.
Conditions:
- System PCD2.M120 with
Firmware 007
- Protocol RTU, 9600 Bauds
- Serial line 1, PGU
active on line 0
- No other program in the
PCD
- The Master is a PCD with
the same Modbus driver
- No other system on the
bus (pear to pear)
- Telegrams: Master
reading (RCV) 16 bits integer values from the Slave
Maximum cycle
time measured on the Slave:
|
232 ms |
|
123 ms |
|
69 ms |
|
42 ms |
|
29 ms |
When the Master
is writing (SEND) values into the Slave, the load is about the half of the
above values.
Critical
speed
In an
uniform and balanced topology (means similar systems on the bus) the
request-response principle will ensure a correct synchronization of all slaves.
Critical situation and synchronization problem can occur in extreme
circumstances. Mixing PCD Slaves with other very fast devices should be
avoided. Fast broadcasting is also a critical practice. In case of problems,
the polling loop must be balanced between devices, the speed must be reduced or
the telegrams size must be reduced.
Timing
problem can occur on systems with a Web application. In this case, reduce the
CPU time allocated to the Web-Server to the minimum. If possible use the
option: Firmware generation = With CSF.
The CPU
load of the driver may also be adjusted through an option in the Fbox MBUS-Mst
and MBUS-Slv. The CPU power used by the driver can be increase but will slow
down the other applications. Note that communication task fully supported by
the firmware have higher priority: PGU port, S-Bus protocol, Profibus or LON
module.
Public
Line Modem
Connection
over public line modem is supported by Master and Slave since version 1.42. The
SAIA Modem library, version $ 2.1.253 or higher must be used.
Leased
Line Modem
Leased
line modem is possible. However test and adjustment with each device may be
needed. Please contact Engiby.
Gateway
Modbus
Slave to S-Bus Master gateway is available with the new Modbus 2 Driver.
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