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Serial I/O Interface Tutorial
Ever since it was introduced, the serial I/O connection, RS-232, has been a source of a great many connection problems. This tutorial, I hope, will shed some light on the problems with serial connections.
First: a TERMINAL always connects to a MODEM. Looking at just one signal, TD (or TXD), meaining “transmitted data,” the question at a connector is always, “Is this an input or an output?” Answer: it depends. Transmitted data is an OUTPUT from the terminal; but transmitted data is an INPUT to a modem. In the old days, the modem would take the data and send it down the phone line. Similarly, RD (or RXD), meaning “received data,” is received by the modem from the phone line. Then RD is an OUTPUT from the modem, and RD is an input to the terminal.
Conclusion: which way a signal is transmitted depends upon whether your end of the connection is a Terminal or a Modem.
Besides the data signals, there are 6+1 more handshake signals: RTS, CTS, DTR, DSR, CD, RI, and GND.
Unless a piece of equipment is a Modem, its serial interface is probably configured as a Terminal. This would include Teletypes, alphanumeric terminals, and personal computers. What happens when you try to connect a PC to an alpha terminal? Well, you can't do it directly. You would be connecting a TERMINAL to a TERMINAL. That violates the first rule above. Something needs to be placed between the two TERMINALs to make each one look like a MODEM to the other. This gizmo is called a NULL MODEM. It is nothing more than a gadget that crosses the appropriate connections, so one input is connected to the other's appropriate output and vice versa. A null modem may be a separate piece of hardware, or it may be a cable whose wires are not wired through from pin X to pin X, but whose wires cross, pin X being connected to pin Y.
The whole situation was very confusing until, AFAIK, DEC standardized its product line.
DEC convention: TERMINAL – connector is a male plug; MODEM – connector is a female socket.
With the IBM PC, the DEC convention was followed: the Serial port on a PC is a Male connector, indicating it is a Terminal connection.
(All olden-day modems had 25-pin female connectors. This may have inspired the DEC convention. Most old alpha terminals have female connectors; but DEC had control over their product line, and they started putting male connectors on their alpha terminals, both hardcopy and CRT. By following this convention, a simple extension cord, M-F cable, can be used to connect a terminal to a modem.)
The N8VEM MultiFunction board brings the serial I/O signals out to a 10-pin header. This header may be wired to an appropriate cable connector, 9-pin or 25-pin, as desired.
BUT THE BOARD MAY BE JUMPERED TO LOOK LIKE A TERMINAL OR A MODEM.
A. Terminal jumpering: (see Jumpered-Terminal.JPG) For connection to an alpha terminal using a NULL MODEM CABLE, the board would be jumpered as shown, as indicated on the silkscreen. The cable does the crossing of signals to make the 25-pin connector look like a modem. Hence, this connector may be plugged into a terminal.
B. Modem jumpering: (see Jumpered-Modem.JPG) For connection to terminal with a DB-9 connector, this is the simplest cable to constuct. The 10-pin female IDC connector which plugs into the board is crimped to the 10-conductor flat cable, and wires 1-9 are crimped into the female DB-9 socket. Wire 10 is not connected. The board is jumpered to look like a modem, per the photo and the silkscreen. The DB-9 connector is Female, indicating it is a MODEM connection. Any terminal may plug into this “modem.”