Standard Modbus Interface for the Thermometers of the ALPI Cryogenic Control System
J. Vasquez1,2, S. Canella1, P. Modanese1, A. Calore1, T. Contran1, A. Friso1, D. Giora1, M. Pengo1
1
2
INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy.
Dipartimento di Ingegneria dell'informazione dell’Università di Padova, Padova, Italy.
INTRODUCTION
In the cryogenic control system for the ALPI cryostats at
LNL there is a large number of thermometers used as
diagnostic devices. During the development of this system,
diverse kinds of thermometers have been installed, each
one with its own proprietary communication protocol, over
different types of interfaces.
During the last years, a new control system for the
cryogenic system has been developed [1]. The
management of this large and heterogeneous thermometer
network on the new control system has become a problem
difficult and expensive to solve.
On this report the adopted solution is presented: a new
standard Modbus interface for all LNL thermometers.
THE THERMOMETERS
The thermometers used in the cryogenic control system
are: (1) Cryocon 18 with an Ethernet interface and 8
temperature channels; (2) SI-9300 with a serial RS232
interface and 8 temperature channels; and (3) SI
Twickenham and CRYOGENIC both with a serial
interface and 6 temperature channels.
On the Modbus server the temperatures are mapped as
32 bits float values (standard IEEE-754) onto two Modbus
registers. The diagnostic status information, on the other
hand, is mapped into bits of Modbus registers.
The device can read up to 6 thermometers
simultaneously, of any kind and combination, each with up
to 8 temperature channels. The number and type of
thermometer is configured using text files on the RPi. If
necessary, the device ca be remotely managed through a
SSH connection.
CURRENT IMPLEMENTATION
Currently four prototypes are under test at LNL, with all
types of thermometers. The Modbus client is a Schneider
Premium PLC. The four RPis read the temperature from 10
thermometers (of different types) and then the PLC can
read the values from each RPi using Modbus.
Figure 1 shows a picture of two of these prototypes.
THE INTERFACE DEVICE
The new device is capable to communicate with each
type of thermometer reading the temperature values and
then share these values with the control system using the
protocol Modbus over TCP. Modbus was chosen because
the new control system is being developed using Schneider
PLCs (Programmable Logic Controllers) which natively
implement this protocol, simplifying the integration.
The core of the device is a Raspberry Pi (RPi), model B,
rev. 2. It has an integrated Ethernet port to communicate
with the PLC and the thermometers with Ethernet
interfaces, while USB to serial port expansions (FTDI
USB-COM232-PLUS4) are used to communicate with the
thermometers with serial interface.
The device runs the Linux operating system Raspbian.
On top of that, runs a custom software developed in C
which consists of two independent threads: (1) one client
that reads the temperature values from the supported
thermometers using their proprietary protocols; and (2) a
Modbus server that delivers those values to the clients.
Additional diagnostic information about the status of the
communication with the thermometers and the status of
each temperature channel are also available through the
Modbus interface.
Fig. 1. Interface device prototypes under test at LNL.
CONCLUSION
The new interface devices allow a simply transition to
the new cryogenic control system, maintaining the
currently installed thermometers. Furthermore, they
present an almost inexpensive solution because of the
ultra-low cost of the RPis and the fact that it is not
necessary to replace the thermometers.
On the other hand, the fact that the same device can be
configured to read any kind and combination of
thermometer, and the ability to be controlled remotely
makes it a very flexible solution.
[1] P. Modanese et al., Cryogenic systems maintenance and
development in 2012. LNL Annual Report (2012), p.217-218.