Argie Rumann – Developing Telemetry Systems with Reconfigurable I/O
One of the biggest challenges in using RIO is that he’s not able to control the clock at a high resolution. Even if he compiles at a 120MHz, the jitter in the clock because of the logic (when he’s trying to get a specific timing) forces him to resort to external timing sources. The problem is that with the shuttle telemetry application they have 6-8 clocks derived from a master clock. They want to clock 2 to the 23 Hertz (in powers of 2). That’s a challenge to bring signals out of the RIO that has the right clocking capability. There’s a need for timing capability with RIO to make explicit the timing characteristics of a signal.
Argie finds RIO provides some basic architectural rules for building the command frames for telemetry which is frame-based communications. He has found the RIO memory useful with a DMA FIFO for working with the bandwidth between MS Windows and RIO. Before, there was a bottleneck in getting the telemetry from the RIO to the front panel. They have 600 byte frames which update seamlessly, when run at 1 Megabit. They are called NASCOM blocks. It’s the standard for Shuttle payload telemetry.
One challenge remaining is that he is mimicking 422 channels. It usually buffers up the signal in the chip with the comparator inside the chip working at the electrical level. He has taken the contiguous digital signals in the RIO and wrote the differential code inside the RIO as opposed to doing it in a circuit. Impedance matching is a problem. The buffers in the RIO need to be configured to handle it safely given the current levels. The fact that there is a direct connection to the channels means there’s no way to actually put buffers on the board and most of the applications seen in communications are differential in their signal input. If he could put resistors inside the RIO so he could pick 422 differential lines to ground, for impedance matching, that would be really helpful. Right now, he has to make some intricate connections inside the wires in the bundle which can cause reflections.