I am trying to connect Unitronics Jazz2 PLC (JZ20-R16) and an Arduino Uno using Modbus protocol, over RS232 interface. I've got all the required connectors for both Arduino (TTL-RS232 MAX3232 converter as the one below) and Unitronics (JZ20-PRG with JZ10-22-CS10 RS232 connector).
PLC is confogured to be Master, Arduiuno – Slave. Configuration is 8N1. Both units operate very well while connected to ModbusSimulators (from modbustools.com). I connect these devices to the PC using a RS232-USB converter.
The problem is that the PLC require DTR and DSR signals. As you can see from the schematic, the TTL-RS232 converter I've used can not provide such signals.
Here's the screenshot of succesfull data stransfer between the PLC and a PC simulator: 
As You can see, CH3/4 (DTR/DTS) are required to be high in order for PLC to communicate over the modbus.But, as I've daid, the TTL-RS232 converter can not give high DTR/DTS signals. Any idea if there is a chance to tweak this ? I've tried to hook DTR/DTS pins of the PLC to +5V , no luck.
Best Answer
Does the PLC and Arduino have the same ground level?
What does the signals look like on the oscilloscope when PLC and Arduino try to communicate?
In other words - are you sure that the DTR/DTS lines are the cause for the problem? Try to replicate the scenario that is seen when you connect the units to the PC, probe the communication lines, and check if they look the same.