SRS Labs SR510 - Manual (Page 44)

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transmitted (11bits/19.2K baud.) The typical data
string 5.1270<cr> has 7 characters, requiring 4
msec to be sent.
Stop Bits
Generally, selection of 2 stop bits will result in
fewer data transmission errors.
Parity
Parity
The Parity bit provides a check against faulty data
transfer. It is not commonly used in local data
transmission environments. If the parity option is
selected, the SR510 will transmit 8 data bits and a
parity bit, however, no parity check of incoming
data is done.
Voltage Levels
The RS232 uses bipolar voltage levels:
The control lines use positive logic. For example,
the DCE tells the DTE that it is clear to send (CTS)
by placing > +3 VDC on pin 5 of the interface.
Similarly, the DTE can tell the DCE that it is not
ready by placing -3 VDC on pin 20 (DTR) of the
interface.
The data lines, pins 2 and 3, use negative logic. A
'zero' bit is represented by a positive voltage and a
'one' bit is represented by a negative voltage. A
start bit is a positive voltage and a stop bit is a
negative voltage. Data is transmitted with the
least significant bit first. The letter 'A', which has
the ASCII code 41H (0100 0001), would appear as
follows:
If a parity option was selected, the parity bit would
be sent after the 8th data bit, but before the first
stop bit.
Final Tip
When you are trying to get the RS232 to work with
your computer, it is helpful to be able to
'eavesdrop' on the RS232 data lines going
between the SR510 and the computer. This can
be done with an ASCII RS232 terminal and the
following connector:
To test the connector, place the hook clip on pin 2
of the same connector (shorting pin 2 to pin 3.)
Now, when you type at the terminal keyboard,
data transmitted from pin 2 is received at pin 3 and
displayed on the terminal screen. To use as a
debugging tool, attach the hook clip to either pin 2
or pin 3 of the RS232 cable on the SR510 to show
either data sent from the Computer or the SR510.
The baud rate, parity, and stop bits of the terminal
must match those of the SR510 and the computer.
If your terminal has a mode which will display
control characters (such as carriage returns and
line feeds) it is helpful to operate in that mode.
A variant of the 'eavesdropping' approach is
diagrammed below:
With this cable arrangement, the ASCII terminal
can listen to the data passing in both directions.
The only drawback is that the terminal will display
garbled data if both devices transmit data at the
same time.