The Acromag Modbus-RTU modules contain non-volatile downloadable flash memory and EEPROM to preserve network & module parameters upon loss of power. If you are experiencing a loss of communications after a recycling of power, here are some things to check:
1) In the Acromag Configuration Software for Series 900MB, make sure the "Update Communications settings at download" box is checked in the Communication Settings window when changing the communication settings of the module. This will keep the communication settings of the Modbus Configuration Software in sync with the communication settings of the device.
2) Make sure the Modbus Function Code you are using is correct. For example, one application was using a Unitronics Vision 230 PLC/HMI package as the modbus master had Modbus Function Code 16, or Preset Multiple Registers programmed into it. This fuction code can write to holding registers 40001 through 40003 changing the slave address, baud rate, and parity. These changes take effect on the next power on-reset of the modbus i/o module. So, upon power recycling, the changes to the holding registers were made and communication was lost. The solution was to change the Modbus function code to 04, or Read Input Registers, which simply reads the contents of the input registers (3xxxx reference registers) in the slave device. No effect on slave address, baud rate or parity settings. Upon testing the network with recycling power multiple times, network communication was established each time.