Using signal routing software to safely program interconnected PXI switch modules
Author : Matthias von Bassenheim, Germany MD & Bob Stasonis, Sales & Marketing Director, Pickering Interfaces
01 May 2020
Pickering 2020 PXI article header image_580x280
Programmable switching systems are routinely used in automated test equipment (ATE) to allow the flexible connection of devices-under-test (DUTs) to measurement & stimulus instrumentation, power supplies, loads & other devices, such as sensor simulators. The increasing functionality of electronic products & the common practice of testing multiple DUTs simultaneously are driving the complexity of switching systems ever higher.
This article was originally featured in EPDT's 2020 PXI for T&M supplement, included in the May 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
The more complex switching systems are, then the more difficult the development of safe and secure switching system software becomes. Even with simple switching tasks, it must be assured that short circuits or incorrectly programmed switching which could potentially damage the DUT or the test system itself are avoided. As Matthias von Bassenheim, Managing Director at Pickering Interfaces GmbH & Bob Stasonis, Sales & Marketing Director, Asia at Pickering Interfaces tell us, compact, yet complex switching systems can be created using low-cost, space-saving PXI cards, and automatic signal routing software will guarantee the safe programming of all interconnected switch modules.
Take, for example, a test engineer needing a high current matrix for a PXI test system. The exact configuration is an 8 x 16 matrix, capable of switching up to 250 VAC and 10 amperes. Because of the size limitations of available PXI switch modules with the required voltage and current specifications, the matrix will require multiple interconnected modules. In this example, we are using the Pickering Interfaces’ 40-551-101 4 x 8 power matrix. The application requires four of these modules with their X and Y connections wired together, as shown in Figure 1.
Now on the surface, this does not appear to be a very complex circuit. But the fact that the four modules must be programmed separately means that the test engineer must be careful in coding the use of this matrix. You will also note multiple signal paths have been programmed at the same time, so taking care not to short active signals together is imperative.
A typical approach to functional test system software development is to use a spreadsheet, in which all the individual test steps are listed, together with their associated instrument parameters and required signal paths through the switching system. In-depth knowledge of the entire switching system is essential. For example, which relay at which physical address has to be switched, at what time and in which test? What is the current state of all other relays; will any switched paths interfere with each other? At the end of this process, the test engineer potentially switches many sets of relays to achieve the required signal paths for the next section of the test. Shouldn’t it be the other way around? In other words, the user asks for a signal path, and the system switches what is required?
For simplicity’s sake, a more straightforward switching system is shown in Figure 2. The five single-pole, single-throw relays on three separate switch modules are wired together. This configuration allows many signal path variations between the endpoint connections M, N, O & P to be set. The normally closed contacts K4 and K5 must be carefully considered as in idle mode when all relay coils are deactivated, these contacts are closed. Unwanted shorts to other nodes in the system can, therefore, easily occur due to incorrect relay operation, and thus this system configuration requires the utmost care in programming.
Figure 3 shows three independent signal paths to be switched sequentially:
Figure 1. Four 10A Power EMR Matrix – 40-551-101 configured as an 8 x 16 matrix
1. signal path M to N (indicated in blue)
2. signal path M to O (indicated in red)
3. signal path M to P (indicated in green)
What is the process and what has to be considered?
1. Path M-N (blue) Start Condition = RESET = all normally open contacts open, all normally closed contacts closed
a. CLOSE K1
2. Path M-O (red) - K1 still closed
a. OPEN K4 - short to N must be avoided
b. OPEN K5 - short to P must be avoided
c. CLOSE K3
d. CLOSE K2
Figure 2. Straightforward switching system & Figure 3. Block diagram with given signal paths
3. Path M-P (green) - K1, K2 & K3 are closed
a. OPEN K2
b. CLOSE K5
4. RESET - all normally open contacts open, all normally closed contacts closed
The above scenario only works without any unwanted shorts if the listed sequence of relay settings takes place.
If a relay path needs to be set without any knowledge of previous switch system settings, a safe condition must be established before the required path is set, for example:
Path M to P (green)
a. RESET
Figure 4. SPM system configuration with modules addressing
b. OPEN K4 - avoid short to N
c. OPEN K5 - avoid short to P
Now all leads are "isolated" = disconnected from any possible connection
d. CLOSE K1
e. CLOSE K3
f. CLOSE K5
This simple example illustrates the programming complexity of a simple switching system – and why a detailed knowledge of the whole system is required. If a path is switched, and another one is added without safety measures, short circuits are unavoidable.
What happens when using signal routing software?
Pickering Interfaces’ signal routing software, “Switch Path Manager” (SPM) creates a virtual image of the switching system architecture and uses this at test program runtime to switch the required signal paths. The user simply configures SPM up-front, by listing all the switching modules used in the system together with the physical connections between these modules (see Figure 4 and Figure 5). Finally, the user defines the endpoint connections (see Figure 6).
Figure 5. SPM virtual wiring list
Endpoints are nodes at the boundary of the switching system, which are connected to the test instrumentation and the DUT. Once the SPM configuration is complete, the user simply needs to regard the switching system as a “black box”, (see Figure 7) without having to worry about internal relays, their addressing and interconnections.
To set a signal path, the user simply selects the desired starting endpoint and target endpoint(s) to be connected, and calls this via an SPM “CONNECT” command. The specific relays to be switched are then determined by SPM. The SPM signal router will always avoid conflicts with existing paths or non-isolated endpoints, and if necessary will find an alternative route or, in the case of an unsuccessful search, abort the process with an appropriate error message.
The sequence of test steps using SPM
1. Path M-N (blue)
a. DISCONNECT ALL
opens all normally open contacts, closes all normally closed
b. CONNECT M, N
2. Path M-O (red)
a. DISCONNECT ALL
Figure 6. SPM endpoints definition
b. CONNECT M, O
If there would be no DISCONNECT ALL at the beginning, the SPM Endpoint Isolation would have intervened on a CONNECT M, O due to the NC contacts to N and P. A connected signal (to N and/or P) would automatically be detected as not isolated (not disconnected), the router being blocked and the path (M to O) not being switched.
3. Path M-P (green)
a. DISCONNECT ALL
b. CONNECT M, P
If a path is switched, another path can only be switched if there are no conflicts with the existing path, for example:
Possible
1. DISCONNECT ALL
2. CONNECT M, N
3. CONNECT O, P
Figure 7. "Black Box" & Figure 3. "Black Box" signal paths
Impossible
1. DISCONNECT ALL
2. CONNECT M, N
3. CONNECT O, N -The command would not be executed and an error message would be returned
Point-to-multipoint connection and disconnection are also possible:
1. DISCONNECT ALL
2. CONNECT M, N+P - A wanted connection, M connected to N AND P
3. DISCONNECT M, N - The previously connected N will be disconnected from M, Connection M-P remains
Conclusion
Even with small switching systems, program development errors are possible and these have the potential to damage the test system hardware and/or DUT. It is therefore always advisable to consider implementing switch routing software, which provides the benefits of reduced program development times and zero programming errors.
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