83
Supported Protocols
Communication with a Modbus RTU or Modbus TCP
master device is possible if the appropriate commu-
nications module is fitted in option slot A. An RS485
Module is required for Modbus RTU. An Ethernet Mod-
ule is required for Modbus TCP. The instrument can
also act as “setpoint master” over RS485 for multi-zone
applications. In this mode the unit continuously sends
its setpoint value using Modbus broadcast messages.
Master mode is not available with Ethernet module.
To protect the EEPROM from excessive write opera-
tions, the 6 most recent parameter write requests are
held in standard RAM. All data is written to EEPROM at
power-down or if another parameter is changed. Avoid
continuously changing more than 6 parameters.
All models also have a configuration socket for bench
setup via the PC configuration software prior to in-
stallation. An RS232 to TTL lead (available from your
supplier) is required in order to use this socket. A
front mounted USB port is available on some models;
this can also be used to configure the instrument or
to transfer recorder or profile files via a USB memory
stick.
RS485 Configuration
The RS485 address, bit rate and character format are
configured via the front panel from the Comms Con-
figuration sub-menu or by using the PC Configurator
software.
Data rate: 4800, 9600, 19200, 38400, 57600 or 115200
bps
Parity: None (default), Even, Odd
Character format: Always 8 bits per character.
Device Address: See below.
RS485 Device Addressing
The instrument must be assigned a unique device ad-
dress in the range 1 to 255. This address is used to rec-
ognize Modbus queries intended for this instrument.
With the exception of globally addressed broadcast
messages, the instrument ignores Modbus queries that
do not match the address that has been assigned to it.
The instrument will accept broadcast messages (global
queries) using device address 0 no matter what device
address is assigned to it. No response messages are
returned for globally addressed queries.
Ethernet Configuration
For Modbus TCP communications (Modbus over Eth-
ernet), the Ethernet IP address can either be assigned
by a Dynamic Host Configuration Protocol (DHCP),
BootP or AutoIP server on the network, or manually as-
signed using the IP address allocation software tool.
Refer to the PC Software section of this manual on
page 235 for more information about setting the IP ad-
dress. The supported data rates 10/100BASE-T (10 or
100 Mbps) are automatically detected.
Link Layer
A query (data request or command) is transmitted from
the Modbus Master to the Modbus Slave. The slave
instrument assembles the reply to the master. This in-
strument is normally a slave device. It can only act as
a master when being use as setpoint master controller
to broadcast its setpoint to other controllers in a multi-
zone application.
Figure 62. Modbus Link Layer
MODBUS
MASTER
SLAVE
INSTRUMENT
QUERY
RESPONSE
A message for either a QUERY or RESPONSE is made
up of an inter-message gap followed by a sequence
of data characters. The inter-message gap is at least
3.5 data character times - the transmitter must not
start transmission until 3 character times have elapsed
since reception of the last character in a message, and
must release the transmission line within 3 character
times of the last character in a message.
Note:Three character times is approximately 0.25ms
at 115200 bps, 0.51ms at 57600 bps, 0.75ms at 38400
bps, 1.5ms at 19200 bps, 3ms at 9600 bps and 6ms at
4800bps.
20 Serial Communications
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