GPIB Expanders and GPIB Isolators Information

GPIB expanders allow the system to interface with more than the maximum number of devices specified in the IEEE 488.1 Standard. According to this Standard, a GPIB system cannot interface with more than 14 electrical devices or peripherals. GPIB bus expanders have two ports. When these ports are connected to two GPIB systems, the expander buffers the signals and allows them to repeat back and forth between the buses. At each port the expander can control up to 14 devices, so the maximum number of devices or peripherals that may be connected to the system is effectively doubled to 28. 

GPIB expanders may also be used to extend the maximum distance between devices.  The IEEE 488 Standard sets 20 meters as the maximum distance between peripherals, but the addition of an expander can extend this to 40 meters, as long as there is a load-carrying device applied for every 2 meters of cable. The goal with any network is to create a system that does not require any additional configuration due to the expander.  There are many expander designs that support this type of performance.  Additionally, GPIB expanders should not affect the speed of the systems on the expander’s side of the system, or reduce its transfer rate.  However, propagation delays are possible through GPIB expanders when data must be transferred between the systems.  When an expander connects two or more GPIB networks, signals transferring from one side to the other experience speed degradation (time delay) when going through the expander.  After all, the network is being asked to function with more peripherals and at a higher system drain than originally designed.  While this type of performance delay is a normal and expected function of the enlarged system, higher-grade expanders may help to minimize the delay.

If the system requires more than the 28 devices, additional GPIB expanders may be added to the system.  This installation methodology, called cascading, allows the system to be expanded to accommodate many additional peripherals.  While there is no functional limit to the number of GPIB expanders that may be cascaded, significant decreases in network stability may occur if great care is not taken when configuring the system.  Even so, some performance and speed degradation should be expected.  However, not all GPIB expanders are cascade compatible, and those that are, often have restrictions on how many expanders can be cascaded.  When in doubt, check with the manufacturer.  Cascading also allow the distance between peripherals to be extended, via extra cabling.