Yes, it is possible to utilize one pair of 24 VDC DI and DO channel to construct half duplex or full duplex serial communication with other pair of DI and DO channel. In your case, channel pair I will be belong to Siemens S7-300 and the other channel pair will be belong to SB SLC-500. one DO channel of S7-300 will be connected to one DI channel of SLC-500 to convey data from S7-300 to CLS-500, while one DO channel of SLC-500 will be connected to one DI channel of S7-300 to convey data from SLC-500 to S7-300.<pre>
| S7-300 | |SLC-500 |
| DO o------|--------------------------------------|---o DI |
| DI o------|--------------------------------------|---o DO |</pre>
For software implementation, from my previous experience, the serial asynchronous communication method implemented is 13 data bits (9 data bits + 4 bit check sum for error checking) from the transmitting PLC to the receiving PLC in 4.1 seconds. The bit rate is 10 bits/second.
But, the detailed programming for this communication method depend on the PLC used, my previous implementation based on AB controllogix, for Siemens S7-300, I don't have idea
