Actisense NDC-4, Install & User Manual

Page 6

© 2015 Active Research Limited

Software Updates

The  NDC’s  built-in  firmware  is  held  in 

‘flash’

 memory, 

allowing quick and easy upgrades using a simple Microsoft 

Windows™ NT/2000/XP/Vista/7 user interface program 

(ActiPatch) running on a connected PC.

It is our policy to provide these updates free on the 

Actisense website (www.actisense.com)

. This upgrade 

can be performed with the unit completely in-situ, via a PC 

connected to 

either

 Port 0 (OPTO IN 0 and ISO OUT 0) or 

the USB port.

Connecting devices together

The Basics

NMEA data is transmitted from an information source such 

as GPS, depth sounder, gyro compass etc. These data 

sending devices are called 

“Talkers”

.

Equipment receiving this information such as a chart-

plotter, radar or NMEA display is called a 

“Listener”

. 

Unfortunately, only one Talker can be connected on to a 

single NMEA 0183 system at any one time. Two or more 

Talkers  are  simply  not  possible  because  they  are  not 

synchronised to each other, and will attempt to ‘talk’ at the 

same time (over each other), resulting in corruption of the 

NMEA  data,  and  potentially  in  disaster  if  valuable  data 

such as navigation information is lost or corrupted so that 

it is incorrect and/or misleading.

Actisense

 produces a full range of products to solve all 

NMEA interfacing requirements.

Please visit the

 Actisense website (www.actisense.

com)

 for full details on these and other 

Actisense

interfacing, Depth sounding and NMEA 2000 products.

The NMEA Signals

The NMEA 0183 system v2.0 and later uses a ”differential” 

signalling scheme, whereby two wires are used to transmit 

the  NMEA  data.  These  connections  will  be  labelled  as 

either NMEA 

‘A’

 and 

‘B’

 or NMEA 

’+’

 and 

‘-’

 respectively, 

depending on the instrument and manufacturer.

When connecting between different manufacturers, there 

can  be  some  confusion,  but  it  is  simple  and  easy  to 

remember: NMEA 

‘A’

 connects to NMEA 

‘+’

 and NMEA 

‘B’ 

connects to NMEA 

‘-’

.

The Different NMEA Standards

The NMEA 0183 specification has slowly evolved over the 

years, so connecting one device to another is not always a 

straightforward matter. The earlier versions of NMEA 0183 

(before  v2.0,  as  detailed  above),  used  slightly  different 

connection methods and signal levels: the instruments 

had just one “NMEA” data line (‘Tx’ or ‘Out’), and used  the 

ground as the other line - similar to the way a computer 

serial port works. This connection method is referred to as 

“single ended” instead of the “differential” method used by 

NMEA 0183 v2.0 devices.

The data format is largely the same between both systems, 

with v2.0 adding some extra sentence strings, and 

removing older (redundant) sentence strings from the 

specification. The situation is further complicated, as many 

manufacturers still use the old (“single ended”) method of 

connection because it is cheaper to implement.

So how can an older type NMEA device be connected to a 

newer type device?

Care is needed – it is possible to damage or overload the 

output of a newer differential device if it is incorrectly 

connected to an older device. This is because the older 

devices used ground as the return, whereas the newer 

devices actually drive the NMEA 

‘-/B’

 line between 5v and 

0v. Thus, connecting this output to ground will result in 

high  currents  being  drawn  by  the  driver  instrument, 

resulting in potential overheating and damage to the driver 

circuits.

However, the new 

Actisense

 ISO-Drive technology allows 

the  user  not  to  worry  about  this  potentially  damaging  

incompatibility.  Instead,  an  ISO-Drive  output  can  be 

connected  to  an  old  type  single-ended  system,  by 

connecting the NMEA 

‘+/A’

 output from the ISO-Drive to 

the single-ended NMEA 

‘Rx’

  or  “In”  input  of  the  device. 

Connect the NMEA 

‘-/B’

 output of the ISO-Drive to the 

ground of the single-ended device. This provides the 

required data signal return current path.

To connect an old type single-ended device to an OPTO 

IN input, connect the NMEA 

‘Tx’

 or 

‘Out’

 output from the 

single-ended driver to the OPTO IN 

‘+/A’

 input of the NDC-

4. Connect the ground line of the single-ended output 

device to the OPTO IN 

‘-/B’

 input of the NDC-4. This 

provides the data signal return current path. If the NMEA 

‘-/B’

 input is left floating, then data corruption / errors may 

occur.

Please refer to 

“Connecting to NMEA devices” on page 

8

 for examples of these connection methods.