Teledyne Carbon Monoxide Analyzer 300 User Manual download pdf (Page 118)

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.

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Teledyne Carbon Monoxide Analyzer 300 User Manual Page 118