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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.
6.3.04 Choice of Number of Data Bits
For most transmissions, the model 7270 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.
The binary dump commands, used for the fastest data transfer rates from the internal
curve buffers, require 8 bit transmission.
6.3.05 Choice of Parity Check Option
Parity checks are not required at the baud rates available in the model 7270 with
typical cable lengths of up to a few meters. Therefore no software is provided in the
model 7270 for dealing with parity errors. Where long cables are in use, it may be
advisable to make use of a lower baud rate. The result is that any of the parity check
options may be used, but the no-parity option will result in slightly faster
communication.
Where the RS232 parameters of the terminal or computer are capable of being set to
any desired value, an arbitrary choice must be made. In the model 7270 the
combination set at the factory is even parity check, 7 data bits, and one stop bit
(fixed).
6.3.06 Auxiliary RS232 Interface
The auxiliary RS232 interface allows up to sixteen model 7270s or a mixture of
compatible instruments to be connected to one serial port on the computer. The first
instrument is connected to the computer in the usual way. Additional instruments are
connected in a "daisy-chain" fashion using null-modem cables, the AUX RS232 port
of the first to the RS232 port of the second, the AUX RS232 port of the second to
the RS232 port of the third, etc. The address of the lock-in amplifiers must be set up
from the front panel before any communication takes place. At power-up the RS232
port of each instrument is fully active irrespective of its address. Since this means
that all instruments in the chain are active on power-up, the first command must be
\N n where n is the address of one of the instruments. This will deselect all but the
addressed unit. When it is required to communicate with another instrument, send a
new \N n command using the relevant address.
6.3.07 Handshaking and Echoes
A handshake is a method of ensuring that the transmitter does not send a data byte
until the receiver is ready to receive it.
In the RS232 standard there are several control lines called handshake lines (RTS,
DTR outputs and CTS, DSR, DCD inputs) in addition to the data lines (TD output
and RD input). However, these lines are not capable of implementing the
handshaking function required by the model 7270 on a byte-by-byte basis and are not
connected in the model 7270 apart from the RTS and DTR outputs, which are
constantly asserted.
Note that some computer applications require one or more of the computer's RS232
handshake lines to be asserted. If this is the case, and if the requirement cannot be
Chapter 6, COMPUTER OPERATION
6-3
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