The VT-MODEM-3 has an RS422 / RS485 port in addition to its RS232 port. Here are some of the capabilities and limitations of this modem:
The RS422 / RS485 port can be connected to a single RS422 device or a network of 2 wire or 4 wire RS485 devices that don’t require handshaking. The RS422 / RS485 port provides TD, RD and GND signals only. The VT-MODEM-3 does not provide an RTS signal for RS485 devices that require transmitter keying, such as radio modems.
Either the RS232 port or the RS422 / RS485 port can be enabled using the VT-MODEM Setup Wizard. If the RS422 / RS485 port is the disabled port, it will not receive or send data. If the RS232 port is the disabled port, it will not receive or send data but it will accept a configuration download from the VT-MODEM Setup Wizard.
The RS232 port is fully featured, including handshaking signals. When the RS232 port is the enabled port, the VT-MODEM-3 functions as a VT-MODEM-1 Industrial Modem.
The RS422 / RS485 port passes characters real time, without buffering or any lead time delay. In 2 wire RS485 mode the RS485 port will remain in transmit mode for 1.5 mS after receipt of the last character. This lag delay is short enough that the modem will return to receive mode before the 2 wire RS485 device transmits its reply.