Alpha Omega Instruments 800 Series User Manual

Writing a Modbus Application
You need to code messages in eight-bit bytes, with no parity bit, one stop bit (8, n,
1). Negative parameter values must be written in twos complement format. Decimal
places are assumed and are not sent or received in modbus packets. For example,
127.5 degrees is sent and read as 1275 degrees. You must look at the decimal
parameter to determine where to place the decimal point. Parameters are stored in
two-byte registers accessed with read and write commands to a relative address.
Messages are sent in packets that are delimited by a pause at least as long as the
time it takes to send 30 bits. To determine this time in seconds, divide 30 by your
baud rate.
Because changing some parameters automatically changes or defaults other
parameters, use the Complete Parameter Download Sequence table in this chapter
to order write commands.
Using a controller address of 0x00 for a write command broadcasts that command to
all the controllers in the network. This is a powerful feature if all the controllers on a
network use all or most of the same parameters.
Packet Syntax
Each message packet begins with a one-byte controller address, from 0x01 to 0xF7.
The second byte in the message packet identifies the message command: read
(0x03 or 0x04); write (0x06 or 0x10); or loop back (0x08).
The next n bytes of the message packet contain register addresses and/or data.
The last two bytes in the message packet contain a two-byte Cyclical Redundancy
Checksum (CRC) for error detection.
The following pages show detailed information on the packets and the data
contained within them.
Packet format:
nnnnnn nn...nnnn
address
command
registers and/or data
CRC
Nonvolatile Memory
The Series 800/850 TEC Controller is designed with Electrically Erasable
Programmable Read Only Memory (EEPROM). EEPROM has a limitation as to how
many times the data can be saved to any one location in memory. The Series
800/850 TEC Controller’s default is to save all parameters to this EEPROM every time
a parameter is modified. Under normal conditions this memory will last for many
years, however, when writing applications that constantly write new parameters via
RS232/RS485/GPIB, it may become necessary to disable the EEPROM while running
these programs to avoid premature aging of the EEPROM.
ALPHA OMEGA INSTRUMENTS CORP. Instruction Manual
Series 800/850 Thermoelectric Cooler Controller Page 102

Alpha Omega Instruments 800 Series Specifications

General

Brand	Alpha Omega Instruments
Model	800 Series
Category	Controller
Language	English