Right. And even for the DB-9 there are at least two pin assignments. The more common is that of PeeCees, another was used on some MicroVAX console interfaces by DEC. The simplest serial interface uses only RX, TX and ground. For this setup, you must use some kind of software handshake to allow the receiving side to stop the sender before an input buffer overflows. Using this three-wire setup, you can go for ETX/ACK (think I have seen this with some HP machine) or for XON/XOFF (common with DEC and Unix, probably others). The difference is not only with the ASCII characters used, but also with the sequence of events. Details beyond scope here (?). RTS (request to send) and CTS (clear to send) are modem control lines that are often used for handshaking. RTS is the DTE (data terminal equipment) signalling to the modem (DCE, data communications equipment) that it wants to send something, CTS is the modem telling the terminal to do so. DSR (data set ready) and DTR (data terminal ready) are modem and terminal telling each other "I'm here for your service". These are another possible choice for hardware handshake. DCD (data carrier detect) is usually a modem telling the terminal "we're online". With some ports / some setups, this signal must be active before anything can be sent out or received. So, if your connection will work does not only depend on the wires but also on the settings at each end of them. If you can select XON/XOFF handshake, you may be able to go with just RX, TX, and ground, but if you need hardware handshake (many PeeCees), you need to connect the particular signals used with your particular device. Tying DCD to DTR is a loopback to make the terminal "see" a carrier all the time. If the classic "DTR feeds DSR and DCD" setup does not work for you, then maybe DTR is not asserted? - You can check this with a simple voltmeter. By watching the TX line, you should be able to tell if this particular side is talking to the other side at all. Otherwise, you cannot tell if your keyboard input is sent, but not echoed, or if just the echo is not sent. I do know people whose first estimate on the time required to get a serial interface running is always "2 workdays". When answering so, they usually hear a "huh? two whole days???", but they often need that time - and it's not because they don't know their job! Things that come to my mind: the "Commodore" is a PC? - Then it will probably prefer hardware handshake, while the Lantronix may want to use XON/XOFF, so the Lantronix may not assert CTS and/or DSR. Consequently, the PC may not be able to send any data. Can the Lantronix be talked into using RTS/CTS or DTR/DSR handshake (maybe it calls it "modem control")? -- Andreas Freiherr Vishay Semiconductor GmbH, Heilbronn, Germany http://www.vishay.com