For Switching Systems in Automatic Test Equipment, Simple to Complex 
    – Is There an Advantage to Using Signal Routing Software?

    Article - Advantage to Using Signal Routing SoftwareProgrammable switching systems are routinely used in Automatic Test Equipment (ATE) to allow the flexible connection of Devices Under Test (DUTs) to measurement and stimulus instrumentation, power supplies, loads and other devices such as sensor simulators. The increasing functionality of electronic products and the common practice of testing multiple DUTs simultaneously are driving the complexity of switching systems ever higher. The more complex switching systems are, then the more complex 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 that could potentially damage the DUT or the test system itself, are avoided. 

    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: 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 user switches sets of relays to achieve the required signal paths for each test. Shouldn’t it be the other way around, i.e., the user asks for a signal path and a system switches what is required?

    A simple switching system is shown in figure 1. The single-pole single-throw relays on three separate switch modules (or subsystems) are wired together.

    A simple switching system is shown in figure 1. The single-pole single-throw relays on three separate switch modules (or subsystems) are wired together. This configuration allows many signal path variations between the end-point 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. The following scenario shows three independent signal paths to be switched sequentially:

    Switch path manager signal paths switched sequentially

    What is the process and what has to be considered?

            1.  Path M-N   Start Condition = RESET = all normally open contacts open, all normally closed contacts closed
                        a. CLOSE K1 

            2.  Path M-O                            - 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

            3.  Path M-P                             - K1, K2 and K3 are closed
                        a. OPEN K2                     
                        b. CLOSE K5

            4.  RESET                                 - all normally open contacts open, all normally closed contacts close

    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 
                        a. RESET
                        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 shows 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?Signal Routing Software - Switch Path Manager (SPM)

    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. Finally, the user defines the end-point connections.

    Switch path manager system configuration with modules addressing

    Switch path manager virtual wiring list

    Endpoints are nodes at the boundary of the switching system that are connected to the test instrumentation and DUT. Once Switch path manager system endpointsthe SPM configuration is complete, the user simply needs to consider the switching system as a "black box", without having to worry about internal relays, their addressing and interconnections.   

    Black box

    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 
                a. DISCONNECT ALL                                                                                                            
                    opens all normally open contacts, closes all normally closed

                b. CONNECT M, N            

    Switch path manager "Black Box" Signal Paths

            2.  Path M-O      
                a. DISCONNECT ALL
                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          
                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. Examples:

    Possible

      1. DISCONNECT ALL
      2. CONNECT M, N
      3. CONNECT O,P
    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 or DUT. It is, therefore, always advisable to consider implementing Signal Routing Software, which provides the benefits of reduced program development times and zero programming errors.

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