RS232 data output by the attached terminal devices is logically “OR’d” by the modem splitter and then transmitted to the modem. Because of this, only one terminal device may send data to the corresponding modem at a time. If more than one terminal device connected to that modem splitter card sends data at the same time, the electrical signals will be combined causing the data going to the modem to be corrupted. The RS232 control signals output by the terminal devices are likewise logically “OR’d” so that if a control signal is active on any of the terminal devices (RTS for example), the modem splitter will assert that signal to the modem. If that signal is not asserted by any of the attached terminal devices, then the modem splitter will pass thru this inactive state signal to the modem.
This design approach allows the RS232 messages output by the modem to be broadcast to all of the attached terminal devices, and for the appropriate terminal device to respond to each message. The message protocol typically includes an identifier so that the terminal devices are able to identify which messages are addressed to them. When the addressed terminal device is ready to reply, if it uses hardware handshaking it will assert the RTS signal which the modem splitter transmits via it’s logical “OR”circuitry to the modem. The modem will respond by asserting the CTS signal which is then broadcast via the modem splitter back to all the terminal devices including the one that is asserting the RTS signal, thus allowing flow control regardless of which terminal device is active.