Teledyne 300M - Manual (Page 121)

Teledyne API Model 300M CO Analyzer Instruction Manual, 04033, Rev. A
3. Adapters - these change from one type plug (DB-9) to another type plug (DB-25).
They do so WITHOUT changing the 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.
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