Lakeshore Learning Materials 642 - Manual (Page 73)

Lake Shore Model 642 Electromagnet Power Supply User’s Manual
Computer Interface Operation 5-21
5.2 SERIAL INTERFACE OVERVIEW
The serial interface used in the Model 642 is commonly referred to as an RS-232C interface. RS-232C is a standard of
the Electronics Industries Association (EIA) that describes one of the most common interfaces between computers and
electronic equipment. The RS-232C standard is quite flexible and allows many different configurations. However, any
two devices claiming RS-232C compatibility cannot necessarily be plugged together without interface setup. The
remainder of this paragraph briefly describes the key features of a serial interface that are supported by the instrument.
A customer supplied computer with similarly configured interface port is required to enable communication.
5.2.1 Changing Baud Rate
To select the Serial Interface Baud Rate press the Computer Interface key. The first computer interface screen appears
as a prompt for Baud.
Computer Interface
Select With °®
Baud: 9600
Use the S or T key to select 9600, 19200, 38400, or 57600 Baud. The default is 9600 Baud. Press Enter to accept the
new selection and continue to the next setting screen. Press Escape to cancel the new selection and return to the normal
display.
5.2.2 Physical Connection
The Model 642 has a 9 pin D-Subminiature plug on the rear panel for serial communication. The original RS-232C
standard specifies 25 pins but both 9- and 25-pin connectors are commonly used in the computer industry. Many third
party cables exist for connecting the instrument to computers with either 9- or 25-pin connectors. Paragraph 7.10.5 gives
the most common pin assignments for 9- and 25-pin connectors. Please note that not all pins or functions are supported
by the Model 642.
The instrument serial connector is the plug half of a mating pair and must be matched with a socket on the cable. If a
cable has the correct wiring configuration but also has a plug end, a “gender changer” can be used to mate two plug ends
together.
The letters DTE near the interface connector stand for Data Terminal Equipment and indicate the pin connection of the
directional pins such as transmit data (TD) and receive data (RD). Equipment with Data Communications Equipment
(DCE) wiring can be connected to the instrument with a straight through cable. As an example, Pin 3 of the DTE
connector holds the transmit line and Pin 3 of the DCE connector holds the receive line so the functions complement.
It is likely both pieces of equipment are wired in the DTE configuration. In this case Pin 3 on one DTE connector (used
for transmit) must be wired to Pin 2 on the other (used for receive). Cables that swap the complementing lines are called
null modem cables and must be used between two DTE wired devices. Null modem adapters are also available for use
with straight through cables. Paragraph 7.10.5 illustrates suggested cables that can be used between the instrument and
common computers.
The instrument uses drivers to generate the transmission voltage levels required by the RS-232C standard. These
voltages are considered safe under normal operating conditions because of their relatively low voltage and current limits.
The drivers are designed to work with cables up to 50 feet in length.