Teledyne 300 instruction manual Wiring

4.Null modems - here the connector changes the internal wiring so that DTE devices can become DCE or vice versa. The main internal change is swapping pin 2 and 3 so that data is transmitted and received on opposite pins.

NOTE

Null modems can also combine gender changer or adapter features in the design. When making up an adapter cable be careful to note what you are using especially with combination null modem-adapter connectors.

Wiring:

The RS-232 is a point-to-point protocol and as such it specifies a two different wiring schemes depending on if you are originating the transmission or receiving the transmission. In the original spec, modems communicated with terminals and were wired as "Data Communications Equipment" or DCE. Terminals or printers received data from modems and thus were wired as "Data Terminal Equipment" or DTE. As technology has progressed it has become more ambiguous who was DCE and DTE. Teledyne API analyzers are wired as DTE (i.e. like a printer). As can be seen, this presents difficulties if you a hook

a printer to the instrument that is likewise wired as a printer. To help understand the different problems - 3 examples as shown below:

Example 1: Connecting the Teledyne API analyzer to an IBM-PC AT compatible computer.

In this case the PC is wired as DCE and the analyzer is wired as DTE therefore a null modem is not needed. The wiring is "straight through" i.e. pin 1 to pin 1, pin 2 to pin 2, etc. Therefore all you have to do here is adapt the connector on the analyzer (male DB-9) to whatever is on the PC. Make sure none of the adapters have null modems in them.

Example 2: Connecting the Teledyne API analyzer to a serial printer.

In this case both the analyzer and the printer are wired as DTE so a "Null Modem" will have to be inserted in the line to change the wiring to make the analyzer look like a modem (i.e. DCE). Make sure in using your adapters that ONLY ONE null modem connector is used. Null modems can be purchased in DB-9 and DB-25 connectors at each

end.

Example 3: Connecting the Teledyne API analyzer to a modem.

P/N 02163G1 Teledyne API Model 300 CO Analyzer Instruction Manual - Page 112