Diagnosing Seatalk1 (Seatalk, ST1) communication problems

Assuming nothing has changed in the system recently, there are two likely causes of a problem with the ST1 network:

• A cable/connection fault (e.g., ST1 data cable shorted to either 12V or 0V)
• A product that is either faulty or not powered and "pulls down" the ST1 network, preventing other devices from changing the network signal voltage to communicate

Diagnostic Steps

1. Ensure all devices are powered on.
   Older ST1 devices (anything older than ST60+ instruments, ST6002/7002/8002 pilot displays, SPX autopilot course computers) can "pull down" the ST1 network (keep the communication signal voltage high or low) and prevent other devices from communicating. If there are any devices that are turned off, switches that are not activated, or fuses that are blown, this may be the cause of the problem.
   
   
2. Simplify the network and find two devices that CAN communicate, and then build from there.
   This is the main point of this FAQ. It is generally much more time-efficient and effective to start by finding a small, core network that you can be sure works and build from there, rather than starting with a larger, non-working network and trying to get it to work. You need to start simple. To follow this method, you need:

• At least 3 ST1 devices. You cannot do any diagnosis this way with only 2 products.
• At least 2 devices that transmit data of some kind (we call this a transmitter, although it can be a display itself, such as an ST60 Speed instrument)
• At least 2 devices that receive data from the above transmitter (this will be one of a wide range of display products: we call this a receiver)
• A selection of cables so you can change connection points

You can meet these requirements with a variety of combinations of products (and some products can function as both transmitter and receiver, such as a chartplotter), but the key idea is that you need to be able to connect 2 devices together and then look for network data between them to show that part of the network is working. For this, you need a device that sends data (an autopilot course computer, speed/depth/wind instrument with connected sensor, GPS etc.: the transmitter) and a device that can display the data it sends (receiver). Some devices are more useful for this than others: for example, an ST60 Speed instrument does not display data from any other device, but an ST6002 autopilot display can show a wide range of data on its data pages, and a chartplotter can display most or all data in the network.

To start, connect a transmitter and a suitable receiver together (remember that the receiver must be able to display the transmitter's data, assuming the network is working: there is no point in connecting a wind display and course computer together because there is no way to see that they are communicating.) Does the receiver display the transmitter's data (e.g., does a pilot display show course from a course computer, does a wind display show true wind with data from a speed instrument)? If not, one of these products or cables has a problem and you need to replace one of them at a time (transmitter for another transmitter, receiver for receiver or the cable) until you find a combination of products that can communicate.

Once you have two products that you can see share data between them, add products and cables one at a time until you see problems. At that point, you have almost certainly found the culprit, and that product or cable needs to be repaired or replaced (more subtle and intermittent problems are possible, but rare).

Go Deeper

The best tool for diagnosing problems like this is an oscilloscope. Unfortunately, you cannot do much useful troubleshooting with a multimeter because even on an old, low-speed network like ST1, the network traffic is much too fast to show whether it is working or not on a voltmeter. If you are a technical person and have access to an oscilloscope, you should look for a 0V - 12V square wave signal at 4800bps (this is not RS232 / NMEA0183 but will look similar on a 'scope). Here is an example of a healthy ST1 network, using a low-cost PC oscilloscope:

In contrast, here are 3 screenshots of ST1 networks with communication problems caused by devices with faulty ST1 communication chips:

In each case, a faulty device loads the bus and prevents other products from being able to pull the signal voltage cleanly up to 12V.

If you cannot diagnose the problem yourself through a process of elimination, or if you do not have enough products to start this process, you need to hire the services of a technical service technician who can visit the boat with an oscilloscope and some test equipment to find out what is wrong.

ST1 cable colors