The 1700M series
uses the Modbus RTU protocol for communications. The Modbus protocol uses
a master-slave technique, in which only the master device can initiate
transactions. The slave devices respond by supplying the requested data to
the master or by taking the action requested in the query.
The master can
address any slave. The returned messages are considered response messages.
The Modbus protocol
format used by a master consists of a device address, a command function
code which defines the operation to be performed, data required with the
command, and an error checking value. The slave response message contains
any required data and error value. If an error occurs, and exception code
will be generated. The supported master codes are:
|
Modbus Command
Functions |
| Function |
Description |
Application |
| 01 |
Read Coil Status |
Read Coil Status is a digital input such as contact status or contact
closure. |
| |
|
|
| 04 |
Read Input Register |
Read Input Register
in the module. |
| |
|
|
| 05 |
Force Single Coil |
Force Single Coil is used to set or clear a digital output. |
| |
|
|
| 06 |
Preset Single Register |
Preset Single Register is used to temporarily suspend Modbus RTU mode and
return to DGH ASCII protocol mode. |
| |
|
|
| 15 |
Force Multiple Coils |
Force Multiple Coils is used to set or clear digital outputs. |
The DGH 1700M series products use a simple command and response protocol. A
module must be interrogated by a host computer to obtain data. A module will never
initiate a command sequence in order to prevent communications collisions.
A command is initiated with a command prompt, may be a dollar sign ($) or pound sign
(#). Following the prompt a single address character must be transmitted. Each module on a
communications bus must be setup with a unique address. The address is followed by a two
character command. Every command is terminated with a carriage return.
The module response begins with a response prompt, which is an asterisk (*) followed by
the necessary data. Every response is terminated with a carriage return. Linefeeds after
the carriage return are user selectable. See the table below for typical commands and
their respective response messages.
