11
A pair of wires should be run from these pins to the COMMON and NORMALLY open
contacts (i.e. the front and middle terminals) of the relay to be used. Normally this will be
the relay associated with the channel being used for Helium level measurement.
However if a helium autofill system is also in use, another relay may be used for
rundown. Refer to section 9.6, for details on configuring the relays for different
purposes.
3.7
Analogue Outputs
Where none of the channels in an ILM have been configured to use the stepper motor
output lines for controlling autofilling, three of these four lines (bits 0 to 2) are
reallocated to provide pseudo analogue output signals representing the levels on the
three channels. These outputs are intended only to drive a chart recorder or similar for
trend recording. For precision data logging the computer interface should always be
used. The pseudo analogue signal is achieved by alternately pulling the output pin to 0v
and releasing it, with a time dependent waveform, the mean value of which represents
the analogue output. To use the output, a pull up resistor is required to a positive
voltage (not greater than 25V), followed by a passive filter with a time constant of
around 5 seconds. A typical output lead giving analogue outputs in the range 0 to 1V is
shown in circuit CNC6802 at the end of this manual. Note that
no analogue outputs
are available if any channel is configured to use a stepper motor for autofill.
3.8
RS232 Serial Data Line Connections
The bi-directional serial data link from the computer is connected via a 25 way D-socket
on the rear panel. ILM is configured as a DCE with the standard pin outs given below.
The majority of computer RS232 interfaces are configured as a DTE and are fitted with a
25 way D plug. For this type of connector, a simple lead connecting pin 1 to pin 1, pin 2
to pin 2 etc. is all that is required. For computers fitted with a 9 way D plug for RS232,
(AT style COM port), a standard "AT lead" fitted with a 9 way socket and a 25 way plug is
required.
Pin connections at the ILM RS232 socket are:
Pin
Signal Name
Notes
1
FG
Linked to Chassis Ground in ILM
2
TD
Data from Computer to ILM
3
RD
Data from ILM to Computer
4
RTS
Linked to Pin 5 in ILM
5
CTS
Linked to Pin 4 in ILM
6
DSR
Linked to +5V when ILM is powered
7
SG
Linked to Digital Ground in ILM
8
DCD
Linked to +5V when ILM is powered
All other pins are open circuit.
ILM does not require signals to be present on any of the "modem control" lines, RTS or
DTR (pin 20). RTS is looped back as CTS and logic high levels are returned on DSR and
DCD to ensure maximum compatibility with any requirement of the computer.
