Model 2651A High Power System SourceMeter® Instrument Reference Manual Section 2: General operation
2651A-901-01 Rev. A / March 2011 2-85
RS-232 connections
The RS-232 serial port is connected to the serial port of a computer using a straight-through RS-232
cable terminated with DB-9 connectors. Do not use a null modem cable. The serial port uses the
transmit (TXD), receive (RXD), CTS and RTS (if flow control is enabled), and signal ground (GND)
lines of the RS-232 standard.
See Flow control and signal handshaking
(on page 2-83) for the rear-panel connector for the RS-232
interface and the pinouts. The connector location is shown in Communications interfaces
(on page 2-
73).
If your computer uses a DB-25 connector for the RS-232 interface, you will need a standard cable or
adapter with a DB-25 connector on one end and a DB-9 connector on the other. An RS-232 cable is
available from the Keithley Instruments website
(http://www.keithley.com).
The following table provides pinout identification for the 9-pin (DB-9) or 25-pin (DB-25) serial port
connector on the computer.
Computer serial port pinout
Signal* DB-9 pin number DB-25 pin number
DCD, data carrier detect 1 8
RXD, receive data 2 3
TXD, transmit data 3 2
DTR, data terminal ready 4 20
GND, signal ground 5 7
DSR, data set ready 6 6
RTS, request to send 7 4
CTS, clear to send 8 5
RI, ring indicator 9 22
* The Model 2651A does not use all RS-232 signals. See the topic Flow control and signal
handshaking (on page 2-83).
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