For those of you running Windows 3.x, WinTECH Software continues to provide and support the following sixteen-bit applications for testing modbus communications links.

ModScan...
MODBUS Data Point Scanner

ModScan is for use in development or maintenance of multi-vendor systems utilizing the popular PLC communications protocol as developed by Gould Modicon. It allows the PC to operate as a MODBUS master device, polling a connected slave for data. The user may select a range of data to display using standard message types 01-04. Data may also be written to the device using single point write commands, (05 & 06), or multi-point writes, (15 & 16). This is one of those programs that you don't need very often, but when you do, it will more than pay for itself the very first time you use it. This Windows application will allow effective testing of MODBUS Slave devices in either ASCII or RTU modes of operation.

Additional Details.
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ModSim...
MODBUS Slave Data Simulator

Modbus Slave Simulation is now available for testing connections to Master Controllers such as Distributed Control Systems. The sister application to ModScan, will allow the user to configure multiple sets of data points to be made available in either RTU or ASCII transmission modes. The ModSim application supports multiple serial connections to multiple masters as well as simulating multiple slave devices to a common master.