XYCOM XVME-530 Thermocouple input module

The analogue bus was implemented with wire-wrap. In the past, converters were always installed in small series; the manpower available was significantly larger than today so linking hundred of wires by hand never presented a production problem. Nobody seriously tried to define the real needs of the analogue bus and because wire-wrap always fitted the requirements, there was no motivation to create a PCB backplane instead. Today with less manpower, training a technician to follow the cloud of wires with schematics that sometimes do not strictly correspond to the implementation is not acceptable. The last generation of G-64 crates have a PCB backplane.

Flexibility is nice but care must be taken

 or the shear number of variants can become unmanageable. One software developer working alone was in charge of the embedded software for a big portion of the power converters in the PS complex. The parameters for every type of chassis were hard-coded in his programs and stored in EPROM instead of using even basic techniques such as menus or autodetection of hardware. As a result, any slight modification in the hardware resulted in a completely new program, derived by cloning an existing program. For example, for the same type of chassis and functionality a different measurement card was selected due to a new precision requirement, so two programs were developed. A couple of years later the commercial power converters installed in the crates were enhanced with an internal polarity inverter.

The result was four different programs. 

Unfortunately after some years the display had become obsolete and a new one was chosen, leading to eight different programs. Another display change and a new communication protocol for a new experimental area were followed by other enhancements to the measurement cards to be unipolar or bipolar. A new ADC chip replaced an obsolete device but returned its value in twos complement instead of complementary offset binary, and so on. After 18 years working on the project the developer retired leaving 350 3.5” DD floppy disks containing more than 300 different programs! The programs were poorly documented and executables running in operational power converters did not appear to have a corresponding program on any of the floppy disks. The system was very close to being in the state of “Nobody touch anything because at least it works!” that can afflict aging systems, preventing any chance for a smooth transition towards a new architecture. Experience shows that if this happens, the next step is usually a rushed total renovation that costs a lot more money and effort than a progressive upgrade.

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