LEYBOLD COMBIVAC CM 31 - RS 232 C interface; 3.1 Description; 3.2 Interface parameters; 3.3 Initial operation

3 RS 232 C interface
3.1 Description
The levels of the RS 232 C interface are defined as fol-
lows:
Level LOW (L) HIGH (H)
————————————————————————
Voltage range -3 to -25 V 3 to 25 V
Logic state logic 1 logic 0
Level designation Mark Space
The interface may be operated in either of the following
modes:
Printer mode
In this mode the data are output every 10 s via the inter-
face.The instrument itself and the entry of parameters is
controlled via the keyboard.
Remote control mode
Upon request (after having received the first character)
the measurement data are output via the interface. Im-
portant equipment parameters may also be set up via
the interface.
When receiving messages through the interface, proces-
sing of commands entered via the keyboard may be de-
layed (up to 2 s max.). The instrument is controlled via
the computer and when no transmission is in progress
the instrument can be controlled via its keyboard. When
wanting to disable the entry of parameters via the instru-
ment’s keyboard the LOCK
®
-function must be enabled.
Notes
After switching on the instrument, the interface will be set
to the printer mode, i.e. it will output measurement data
every 10 s without a further request to do so. Immedia-
tely after receiving the first character from a connected
control computer the instrument will then change to the
remote control mode.
For the transmission of data, only characters from the 7-
bit ASCII code will be used.
When using a computer, this computer will require an
input buffer size of at least 30 characters.
XON and XOFF handshaking is not used to control the
data flow and will lead to error messages.
The RS 232 C interface requires at least 3 lines:
- Transmit line (TxD; Transmit data)
- Receive line (RxD; Receive data)
- Reference ground (GND; Signal ground)
The connections are made through a 9-way Sub-D
connector (7/2).
25
3.2 Interface parameters
3.2.1 Baud rate
The baud rate is fixed to 2400 baud.
3.2.2 Data format
The data format is fixed to:
1 start bit, 7 data bits + 1 space, 1 stop bit.
A parity bit is not generated and no parity check is run on
received data.
3.2.3 End and acknowledgement character
for remote operation
The character <CR> (carriage return; ASCII code 13
d
) is
used as the end character for remote control operation in
both directions.
After a A-series instrument has received a string of cha-
racters terminated by the end character <CR> it will res-
pond by outputting the ASCII character <ACK> (acknow-
ledge) or <NAK> (not acknowledge) depending on
whether the instrument has sensed the command so that
it can be carried out or not.
3.2.4 Output rate and end character for
printer output
The output rate is fixed to 6 measurement data sets per
minute, i.e. all measurement data or equipment status
messages are transmitted at an interval of 10 s.
In the printer mode the characters <CR> <LF> (carriage
return; ASCII code: 13
d
and line feed; ASCII code: 10
d
)
are used.
3.3 Initial operation
3.3.1 Remote control operation
3.3.1.1 Cable link
In order to link the A-series instrument to a computer or
terminal, a cable link has to be provided according to
Section 3.8.
The A-series instrument requires at least 3 lines :