Re: RS232 multi drop network
I expect that the devices have some addressing capability to assure only one is answering at a time. Some vendors manufacture devices with RS232 interface, that - different from usual devecied - utilizes a tristate driver. They can actually work in a single host multidrop network by simply paralleling all devices. They provide e.g. up to 10 device addresses. Operation is limited to short range, e.g. a laboratory. My customers in the laboratory automation field are using such devices (e.g. VICI mutiposition valve controllers), and they are working quite well to my opinion.
There would be no problem to install a RS485 driver at the device, but then the user needs a special RS485 converter or interface at his PC. It's a kind of poor man's RS485...
Apart from having devices with dedicated hardware, there is an option to simply install 1N4148 diodes in each device's Tx line and a pull-down resistor, at best to a control line at -12V. The solution would be limited due to crosstalk and capacitive load, but is supposed to work as well.