The Relay Barrier in an In-System Programming Equipment

The parametric or In-Circuit Test (ICT) consists on checking the correct values of capacitors, resistors, diodes, and other components, and on the search of unwanted open and short circuits.

This is done through the bed of nails inside the fixture.

The In-System Programming phase uses the same fixture to In-Circuit Program the UUT. It’s extremely important that the In-System Programmer lines don’t introduce unwanted impedances that may affect the ICT measurements.

The In-System Programmers on the market employ two ways for obtaining high impedance on their ISP lines:

Driving them to tri-state (using a solid-state semiconductor like a line buffer);
Using relays to disconnect them from the target system.
Driving ISP lines to tri-state is easier and cheaper to design than the relay approach. On the other hand, the high impedance obtained this way may not be enough to isolate it from the UUT.
Using relays helps keeping a higher Zeq impedance; in particular, the parasitic capacitance is lower than in the above approach (1-5pf), while the leakage current is 0.

The multi-site WriteNow! models have a dedicated line and software instructions for driving an external relay barrier or a demultiplexer module.

Algocraft Relay Barrier Modules & Demultiplexer Modules

Algocraft supplies a series of accessories that have been designed to ease the integration to customers as well as saving on engineering time and costs. These accessories were designed to be integrated with WriteNow! Programming series easily with a full hardware and software compatibility. These modules have been used by our customers since many years.