Experimental Physics and
Industrial Control System

"Mark Rivers" <[email protected]> · Mon, 30 Apr 2007 13:38:39 -0500

Folks,

I have released a new EPICS module called "modbus".  This module is
designed to be a general-purpose interface to Modbus using the asyn
module for low-level drivers and for EPICS device support.

It was initially based on the modtcp and plctcp modules from Rolf Keitel
at Triumf.  The following are the main enhancements of modbus compared
to those modules: 

- modbus supports serial RTU and serial ASCII in addition to TCP. modtcp
supports only TCP. 

- modbus supports most Modbus function codes, including 1, 2, 5, and 15
for reading and writing coils (single bits) and 3, 4, 6, and 16 for
reading and writing registers. modtcp supports only Modbus functions 3
(read holding register) and 6 (write holding register). 

- modbus supports pseudo-interrupt mode on inputs. The poller thread
calls device support when there is a new value, so the EPICS records can
have SCAN=I/O Intr and do not need to periodically read the inputs. This
is more efficient. 

- modbus supports array operations on reads and writes using waveform
records. 

- modbus uses asyn for underlying TCP/IP or serial connections. This
reduces the amount of code, and has good debugging and connection
management. 

- modbus uses standard asyn interfaces and asyn device support. This
reduces the amount of code significantly. 

- modbus should run on all EPICS platforms. It has been tested on
linux-x86, vxWorks, win32-x86 (native Windows with VC++ compiler), and
cygwin-x86 (Windows with gcc compiler and Cygwin library). modtcp only
works on Linux and vxWorks. 

- modbus provides EPICS PVs for PLC status and statistics. 

The following are some drawbacks of modbus compared to the modtcp and
plctcp packages from Triumf: 

- modbus only works on EPICS 3.14, not on 3.13. 

- There is no Siemens PLC support. It should be a separate driver, using
drvModbusAsyn.c as a model. Most code in modbus is Modbus specific, so
it is cleaner to write a separate, but similar driver for other
protocols. 

- modbus has one thread per asyn port, which is a group of Modbus
registers or coils. This is more threads than modtcp, which had only 1
thread per PLC. There is an advantage in the modbus design, because each
can poll at a different rate, can have different Modbus function code,
and a different data type. It also allows much finer granularity in
debugging output. But it will lead to somewhat increased resource usage
compared to the modtcp design. 

The documentation is at:

http://cars.uchicago.edu/software/epics/modbusDoc.html 

and

http://cars.uchicago.edu/software/epics/modbus.html

The distribution is at:

http://cars.uchicago.edu/software/pub/modbusR1-1.tgz 

modbus requires the latest (R4-8) version of asyn, which is available
here:

http://www.aps.anl.gov/epics/download/modules/asyn4-8.tar.gz

Cheers,
Mark

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System