Magnetrol Jupiter JM4, Operating Manual

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41-621.1 HART ® to Modbus ® Adaptor Operating Manual
13.
Auto Switching Between Modbus RTU/ASCII and HART over RS-485
Communication
13.1.
Purpose
This procedure instructs how to configure the HART to Modbus Adaptor (HMA) to automatically switch
between Modbus RTU and HART over RS-485 communication when it receives the appropriate
commands. This procedure uses a Model 706 as the attached device as an example; other Magnetrol
HART devices can also be used with this procedure.
This provides a convenient method for configuring or troubleshooting an attached HART device using
PACTware. The process is to set a register to a value of 1 while the HMA is in a Modbus
communication mode. The HMA will automatically switch to the HART over RS-485 mode. No cycling
of power is required. PACTware can then be connected to a device through the RS-485 terminal block
on the HMA. Once the PACTware session is completed, by sending a HART command 0 with a poll
address of 63, a user can cause the HMA to automatically revert to the previous Modbus configuration
protocol settings. Again, no cycling of power is required.
This procedure demonstrates the auto-switching feature using the Modbus RTU protocol. The process
can be used for the Modbus ASCII protocol by setting the HMA for that protocol in step 13.4.2.
13.2.
Equipment
Item Manufacturer Model
HART to Modbus Adaptor MII 031-2859-001
RS485 Communications cable FDTIchip USB-RS485-WE
Termination resistor - 120

Modbus host application www.modbustools.com Modbus Poll
Power Supply - 20-24V, 0.5A
PACTware PACTware Consortium Version 4.1 or higher
Level transmitter MII Model 706
Probe MII Model 706 compatible
13.3.
Setup
Connect the HMA to a power supply via the power terminal block. Connect an RS-485 communications
cable to the RS-485 terminal block of the HMA, with the Receive/Transmit Data+ A lead (orange) on
the positive terminal and the Receive/Transmit Data
– B lead (yellow) on the negative terminal.
Connect a 120

resistor between the two RS-485 terminal block positions. Connect the other end of
the cable to a PC which has a Modbus host application. Connect a Model 706 transmitter and probe,
or other supported Magnetrol HART transmitter, to the HART loop terminal block of the HMA. The
device’s Poll Address can be set to any value between 0 and 14.
13.4.
Procedure
13.4.1
Using Procedure 1, ensure that registers 3000 through 3007 of the HMA match the values
shown for the Modbus RTU protocol default settings. The address shown in register 3001
can be between 0 and 14. Refer to Appendix F for the HMA Communication settings. If
the settings do not match, double-click on a value that needs to be changed to open the
Write Single Register dialog, enter the new value, and then click on Send.