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Chapter 6, COMPUTER OPERATION
6.3.02 General Features
6.3.03 Choice of Baud Rate
6.3.04 Choice of Number of Data Bits
6-2
commands on a keyboard and reads the results from the display.
The simplest way to establish this terminal mode is to connect the instrument to a
computer running a terminal emulator, such Windows HyperTerminal.
The RS232 interface in the model 7124 is implemented with 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 are shown in appendix B and cable
diagrams suitable for coupling the instrument to a computer are shown in
appendix C.
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 an 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.
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
7124 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
computer. Lower baud rates may be used in order to achieve compatibility with older
equipment or where there is some special reason for reducing the communication
rate.
For most transmissions, the model 7124 lock-in amplifier uses the standard ASCII
character set, containing 127 characters represented by 7-bit binary words. If an 8-bit
data word is selected, the most significant bit is set to zero on output from the lock-in
amplifier and ignored on input.
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