iSMA-B-MG-IP User Manual
www.ismacontrolli.com
DMP260en | 1st Issue rev. 6 | 05/2022
page 13 of 29
The iSMA-B-MG-IP has a built-in Modbus TCP/IP
to Modbus
RTU/ASCII gateway, which
allows to connect Modbus meters or other Modbus devices using the RS485. In order to
ensure proper operation of the Modbus gateway, please make sure that all parameters of
RS485 (baud rate, stop bits, etc.) for all the devices connected to RS485 (gateway and
Modbus meters/Modbus devices) are the same.
Operation of the gateway is based on checking the Modbus address in the Modbus TCP
query. If the address is different from the set on the DIP switch, data frame from the
query is converted to Modbus RTU/ASCII, and sent to the COM1 port. Then, the meter
gateway waits for an answer from the device connected to the COM1. If the slave device
does not respond within the estimated time defined by the RS485 Timeout (by default,
500 ms), an exception response will be given: error code 0x0B. If the slave device answers
the correct frame, it is converted to Modbus TCP, and sent to the master device.
The meter gateway sends an error code 0x0B in case the valid frame is not received.
Upon receipt of this error code, some systems do not repeat the query according to the
settings. No retry may cause the point to immediately go to down mode. That is why the
meter gateway has the ability to block the sending information about errors (Send
Modbus Errors setting in the web page or iSMA Configurator). Then, in absence of any
response, the system retries the request according to the settings.
5.1 Communication
5.1.1 RS485 Connection
Figure 7. RS485 connection
RS485 Network Termination
Transmission line effects often present problems for data communication networks.
These problems include reflections and signal attenuation.
To eliminate the presence of reflections of signal from the end of the cable, the cable
must be terminated at both ends with a resistor across the line adequate to its
characteristic impedance. Both ends must be terminated since the propagation is
bidirectional. In case of an RS485 twisted pair cable this termination is typically 120
Ω.
