DH485 is not just RS485, part MCMXLII
Thank you Corbis, for alluding to the basic issue here.
Most RS-232 to RS-485 converters do a fine job of converting the TX and RX pin signals of an RS-232 serial port into transmissions and receptions on the A/B pair of the RS-485 data bus.
Data Highway 485 protocol requires some very precise transmit-enable behaviour of the RS232 port. This is not handled simply by letting the 232/485 converter sense RS485 data and start transmitting on RS232, but rather by performing a Transmit Enable function on the DH485 plug. Take a look at the wiring diagram on a 1747-PIC box.
This is why there are specific RS232/RS485 converters that are used with DH485 protocol. The A-B 1747-PIC is the oldest and most venerable, and the 1761-NET-AIC converter does exactly the same thing with two serial ports, different cabling, and built-in isolation.
Any RS232/RS485 converter that is advertised to replace the 1747-PIC or 1761-NET-AIC includes similar Transmit Enable circuitry.
An additional wrinkle comes from the difficulty of getting the very old 1747-PIC driver to work with Windows NT/2000/XP serial port drivers, and with modern laptop serial ports. That's why A-B offers the USB/DH485 converter, part number 1747-UIC, so all that stuff doesn't need to be messed with.
Last problem: The 1747-UIC works best with DH485 networks, and does not always connnect simply to standalone fixed-IO SLC controllers or 5/01 and 5/02 controllers. This is a double whammy because these ancient controllers are the ones most likely to be in the hands of someone unfamiliar with DH485 networking.
I present my customers with a simple decision matrix.
1. Does your PC have a built-in RS232 serial port ?
If Yes, I will help you set up and use the RSLinx 2.51 "1747-PIC/AIC+" driver. I will not support older versions of RSLinx nor can I attempt to support add-on RS232 ports.
2. Buy a 1747-UIC and I will teach you how to connect it to an old SLC-500 controller by generating extra network traffic that "kickstarts" the DH485 token rotation.