Signal Output XY Mode

This mode provides a sequence of signal values for transmit using each frame's timing as specified in the database. This is the recommended mode for writing a sequence of all signal values.

Use nxWriteSignalXY for this mode. The timestamp array is unused (reserved).

Each signal value is mapped to a frame for transmit. Therefore, the array of signal values is mapped to an array of frames to transmit. When signals exist in the same frame, signals at the same index in the arrays are mapped to the same frame. When signals exist in different frames, the array size may be different from one cluster (signal) to another.

The frames for this mode are stored in queues, such that every signal provided is transmitted in a frame.

Examples

In this example network, frame C is a cyclic frame that transmits on the network once every 2.0 ms. Frame E is an event-driven frame that uses a transmit time (minimum interval) of 2.5 ms. For information about cyclic and event-driven frames, refer to Cyclic and Event Timing.

Each frame contains two signals, one in the first byte and another in the second byte.

The example uses CAN. The following figure shows a timeline of a frame transfer on the CAN network. Each frame contains its name (C or E), followed by the value of its two signals. The timeline begins with a single call to nxWriteSignalXY.

The following figure shows the data provided to nxWriteSignalXY. The session contains all four signals.

Assuming the Auto Start? property uses the default of true, the session starts within a call to nxWriteSignalXY. This occurs at 0 ms in the timeline. Frame C transmits followed by frame E, both using signal values from the first sample (index 0 of all four Y arrays).

According to the database, frame C transmits once every 2.0 ms, and frame E is limited to an event-driven interval of 2.5 ms.

At 2.0 ms in the timeline, signal values 3 and 4 are taken from index 1 of the first two Y arrays and used for transmit of frame C.

At 3.5 ms in the timeline, signal value 5 is taken from index 1 of the third Y array. Because this is a new value for frame E, it represents a new event, so the frame transmits again. Because no new signal value was provided at index 1 in the fourth array, the second signal of frame E uses the value 8 from the previous transmit.

At 4.0 ms in the timeline, signal values 5 and 6 are taken from index 2 of the first two Y arrays and used for transmit of frame C.

Because there are no more signal values for frame E, this frame no longer transmits. Frame E is event driven, so new signal values are required for each transmit.

Because frame C is a cyclic frame, it transmits repeatedly. Although there are no more signal values for frame C, the values of the previous frame are used again at 6.0 ms in the timeline and every 2.0 ms thereafter. If nxWriteSignalXY is called again, the new signal values are used.

The next example network demonstrates a potential problem that can occur with Signal Output XY mode.

In this example network, frame C is a cyclic frame that transmits on the network once every 2.0 ms. Frame X is a cyclic frame that transmits on the network once every 1.0 ms. Each frame contains two signals, one in the first byte and another in the second byte. The timeline begins with a single call to nxWriteSignalXY.

The following figure shows the data provided to nxWriteSignalXY. The session contains all four signals.

The number of signal values in all four Y arrays is the same. The four elements of the arrays are mapped to four frames. The problem is that because frame X transmits twice as fast as frame C, the frames for the last two arrays transmit twice as fast as the frames for the first two arrays.

The result is that the last pair of signals for frame X (1 and 2) transmit over and over, until the timeline has completed for frame C. This sort of behavior usually is unintended. The Signal Output XY mode goal is to provide a complete sequence of signal values for each frame.

The best way to resolve this issue is to provide a different number of values for each signal, such that the number of elements corresponds to the timeline for the corresponding frame. If the previous call to nxWriteSignalXY provided eight elements for frame X (last two Y arrays) instead of just four elements, this would have created a complete 8.0 ms timeline for both frames.

Although you need to resolve this sort of timeline for cyclic frames, this is not necessarily true for event-driven frames. For an event-driven frame, you may decide simply to pass either zero or one set of signal values to nxWriteSignalXY. When you do this, each call to nxWriteSignalXY can generate a single event, and the overall timeline is not a major consideration.