VersaPro Temperature Controller
Page 48
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17
Communications
17.1 Modbus
The MODBUS protocol describes an industrial communications and distributed control system (DCS) that
integrates PLCs computers, terminals, and other monitoring, sensing, and control devices. MODBUS is a
Master/Slave communications protocol, whereby one device, (the Master), controls all serial activity by
selectively polling one or more slave devices. The protocol provides for one master device and up to 247
slave devices on a half duplex twisted pair line. Each device is assigned an address to distinguish it from all
other connected devices.
The VersaPro recognizes three Modbus RTU (Remote Terminal Unit) commands. These are: read single I
registers (command 4), read a single H register (command 3), and preset a single H register (command 6)
In the RTU protocol sends data in 8-bit binary characters. Message characters are transmitted in a continuous
stream. The message stream is setup based on the following structure:
Number of bits per character:
Start
bits
1
Data bits (least significant first)
8
Parity
0 (no bits for no parity)
Stop
bits
1
or
2
Error Checking
CRC (Cyclical Redundancy Check)
In Modbus mode, the VersaPro can be only be configured for the ‘none’ parity option.
The instrument never initiate communications and is always in the receive mode unless responding to a query.
RTU Framing
Frame synchronization can be maintained in RTU transmission mode only by simulating a synchronous
message. The instrument monitors the elapsed time between receipt of characters. If three and one-half
character times elapse without a new character or completion of the frame, then the instrument flushes the
frame and assumes that the next byte received will be an address. The follow command message structure is
used, where T is the required character delay. Response from the instrument is based on the command.
T1,T2,T3 ADDRESS FUNCTION DATA
CHECKSUM
T1,T2,T3
8-BITS
8-BITS
N X 8-BITS 16-BITS
Address Field
The address field immediately follows the beginning of the frame and consists of 8-bits. These bits indicate
the user assigned address of the slave device that is to receive the message sent by the attached master.
Each slave must be assigned a unique address and only the addressed slave will respond to a query that
contains its address. When the slave sends a response, the slave address informs the master which slave is
communicating.
Function Field
