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8 MODBUS
The Modbus TCP gateway is a feature that allows the DAS to share the attached Modbus RS485 devices
with other systems such as PCs with software and building control systems. The gateway receives
incoming TCP requests, verifies the length of the packet does not exceed 255 bytes, and forwards the
packet to the Modbus RS485 loop. The gateway does not verify the contents of the packet, nor the target
add ress specified in the request. If the Modbus RS485 device sends a valid response back, the gateway
will return this information to the the system requesting it via TCP. If no response is received, a timeout
failure will be sent to the requester. Because the gateway does not process the contents of the request or
reply, it is possible to query devices on the Modbus RS485 loop that are not specifically supported by the
DAS. A Modbus/TCP technical guide may be available soon at www.leviton.com.
Modbus RS/485 baud rate
: This feature allows the configuration of the RS485 serial port speed which
the DAS uses to communicate with Modbus devices. Options include 9600, 19200, 38400, 57600, and
115200. The default is 9600, most Modbus devices communicate at this speed as well. Multiple baud
rates may be selected in this menu. If multiple rates are selected, the DAS will automatically detect the
baud rate of each Modbus device from the chosen options. Once the baud rate has been detected for a
specific Modbus device address, and will continue to communicate with that specific device address at the
detected speed. This feature will allow you to mix multiple devices on the same RS485 loop with different
baud rates. Modbus devices default to a baud rate of 9600. Note: Modbus device manufacturers usually
do not “officially” support this configuration, however we have not seen any problems with this in any of the
Modbus devices we provide drivers for. Most Modbus devices simply ignore packets received at the wrong
baud rate. If you are experiencing Modbus communications problems, you should set the baud rate to a
fixed 9600 before troubleshooting.
Modbus RS485 Parity:
This feature allows alternate parity options to be set. Default is no parity.
Modbus RS485 Stopbit:
This feature allows alternate stop bit options to be set. Default is one stop bit.
Modbus RS485 Timeout:
This feature sets the period of time the DAS will wait for a response from any
device on the Modbus RS485 serial loop. The default of 200ms is long enough for most Modbus devices.
Longer timeouts may be required if you have a very slow device, or if you have a Modbus repeater device in
the serial loop. Devices such as the ModHopper, the Veris H8936 Local Display, or any RS485 wireless link
repeaters add a substantial delay. Setting the delay longer than the default will increase the time required to
auto detect new Modbus devices when attached to the DAS. For ModHopper connections, use a minimum
setting of 1.5 seconds. The allowed range is 100ms to 5 seconds.
Tip: Set the timeout to the longest available setting, and use the “stats” option on the bottom right of the
device list page to monitor the actual RTT (response time) for each device. Next, decrease the RS485
Timeout that is close to (but longer than) the slowest device in the device list.
Modbus RS485 Debug:
This feature records Modbus RS485 transaction data in the system debug log file.
You must click the “start log” button on the system/log files page to record this information. Debug options
are as follows:
•
None: No RS485 information is logged.
• Errors: Only Modbus errors are logged.
• Transaction: One line is logged describing every RS485 query, and the result of that query.
• Full Debug: Complete listing of all queries including hex dumps of all data sent/received.
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