Table of Contents
Advertisement
Chapter 6, COMPUTER OPERATION
6.3.02 General Features
6.3.03 Choice of Baud Rate
6-2
The simplest way to establish this terminal mode is to connect the instrument to a
computer running a terminal emulator, such the free PuTTY program or Windows
HyperTerminal.
The RS232 interface in the model 7270 uses three wires; one carries digital
transmissions from the computer to the lock-in amplifier, the second carries digital
transmissions from the lock-in amplifier to the computer and the third is the Logic
Ground to which both signals are referred. The logic levels are ±12 V referred to
Logic Ground, and the connection may be a standard RS232 cable in conjunction
with a null modem or, alternatively, may be made up from low-cost general-purpose
cable. The pinout of the RS232 connectors is shown in appendix B and cable
diagrams suitable for coupling the instrument to a computer are shown in
appendix C.
Note: A null modem RS232 cable is not simply one with different polarity
connectors at each end (e.g. 9-pin male at one and –pin female at the other). It
actually includes cross-over connections which are critical to successful operation.
Most problems establishing communications via RS232 with
SIGNAL RECOVERY lock-in amplifiers are caused by trying to use a non null-
modem cable.
The main advantages of the RS232 interface are:
1) It communicates via a serial port which is either already fitted or can be added at
low cost via a USB adaptor to any computer, using leads and connectors which
are available from suppliers of computer accessories or can be constructed at
minimal cost in the user's workshop.
2) It requires no more software support than is normally supplied with the
computer, for example Windows HyperTerminal, or available to download free
of charge, such as PuTTY.
A single RS232 transmission consists of a start bit followed by 7 or 8 data bits, an
optional parity bit, and 1 stop bit. The rate of data transfer depends on the number of
bits per second sent over the interface, usually called the baud rate. In the model
7270 the baud rate can be set to a range of different values up to 38,400,
corresponding to a minimum time of less than 0.25 ms for a single character.
Mainly for historical reasons, there are a very large number of different ways in
which RS232 communications can be implemented. Apart from the baud rate
options, there are choices of data word length (7 or 8 bits), parity check operation
(even, odd or none), and number of stop bits (1 or 2). With the exception of the
number of stop bits, which is fixed at 1, these settings may be adjusted using the
RS232 Settings menu, discussed in chapter 5. They may also be adjusted by means
of the RS command.
NOTE: In order to achieve satisfactory operation, the RS232 settings must be set
to exactly the same values in the terminal or computer as in the lock-in amplifier.
Where the lock-in amplifier is connected to a terminal or to a computer
implementing an echo handshake, the highest available baud rate of 38,400 is
normally used if, as is usually the case, this rate is supported by the terminal or
Advertisement
Table of Contents
