4.6
Data Communications with the Watlow Series 988 Family
Sending Commands, Chapter 4
NOTE:
Modbus register
addresses are
listed in the
Controller Prompt
Table later in this
chapter and in the
Modbus RTU
Address Table at
the end of this
chapter.
Modbus Remote Terminal Unit (RTU)
Modbus RTU, available on Series 988 controllers, expands the communications
ability of the controller by enabling a computer to read and write directly to registers
containing the controller’s parameters. With it you could read all 141 of the
controller’s parameters with five read commands.
Because of the wide array of choices available for setting up a Series 988 controller,
only a subset of the prompts contain parameters in a given situation. The 988 User’s
Manual explains the interrelations between prompts. A Modbus read command will
return a parameter of 0 for an inactive prompt. If you try to write to an inactive prompt
the controller will return an illegal data address message (02).
If you already have a software application that uses Modbus, you can simply skip to
the Temperature/process Controller Prompt Table or the Modbus RTU Address
Table in this chapter for the address information your program will need. The rest of
this section on the Modbus provides information for writing a software application that
uses Modbus.
Writing a Modbus Application
You need to code messages in eight-bit bytes, with no parity bit. Negative parameter
values must be written in twos complement format. 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 param-
eters, 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.
Packet format: |
nn | nn | nnnn…
| nn nn |
∆
∆
∆ ∆
∆ ∆
address
command
registers and/or data
CRC
Modbus RTU
