It seems, in view of the ongoing ECDIS installation boom, that lack of a lot of very basic NMEA knowledge is causing severe problems.
First of all, all type-approved bridge equipment serial data inputs are compliant with either NMEA 0183/IEC 61162-1, or, especially AIS/Gyro/autopilot equipment which need high speed 38 400 b/s data, NMEA 0183HS/IEC 61162-2.
Both of these standards, where the high-speed version is also compliant with the standard version, are always always galvanically isolated from any other part of the equipment. This makes it possible to safely connect any NMEA data output to one or more inputs in parallell without causing any interference or malfunction.
A couple of years ago, we got aware of the fact that a lot of bridge installations actually were connecting equipment with standard RS232 COM ports (Rx, Tx, GND) to NMEA networks. This is obviously not allowed on a SOLAS ship and violates any type approval. Not only violating the rulework, it is also bad engineering/installation practice, which can result in loss of data, affect other equipment and cause damage.
As a result of this, we did upgrade the 1N4 buffer to the “B” version. This version got a completely new output stage driver, also galvanically isolated not only from the input but also from the power supply. This makes it possible to maintain serial data integrity even if one of the outputs of the unit is connected to a RS232 input. BUT ONLY ONE!
What we see happening now is that a lot of ECDIS installations where sensors are connected to a second back-up ECDIS is causing various problems and malfunctions. The main problem is probably that as the second ECDIS is connected, the uncompliance with the rulework is no longer formal but a real problem!
Apart from the obvious advice to use only strictly type-approved ports, here is some advice:
- If it is necessary to connect to a RS232/COM/DSub-9 input, this should work to connect NMEA out A – RS232 Rx and NMEA out B – RS232 GND. It violates a number of rules, but will work with plenty of margin. But again – ONLY ONE! Remaining outputs can feed tyep-approved isolated inputs without problems.
- Use a separate 1N4B unit for each RS232 input to comply with the paragraph above.
- If it in necessary to feed more than one RS232 inputs, it gets trickier. Our advice would be to still connect NMEA out A – RS232 Rx but instead connect NMEA out C – RS232 GND. This would violate the same rules, but would still work, however with less margin!
- Connect NMEA out B to side-grounded ports (like RS232) unless you know how it is connected.
- ECDIS and other battery backed-up systems have large 12 or 24 V battery banks, meaning that also small potential differences can cause large currents. We have come across 1N4B units were the parallell PCB path between the NMEA C connections was just vapourized. This could obviously cause problems more severe than loss of data!