To achieve the optimal data transfer rate of GPIB, you must limit the physical distance between devices and the number of devices on the bus. This limitation is necessary because the GPIB is a transmission line system. Any distance beyond the maximum allowable cable length, as well as any excess GPIB device loads, may surpass interface circuit drive capability. The IEEE 488 specification dictates the following limits:
- Maximum separation of 4 m between any two adjacent devices and an average separation of 2 m over the entire bus
- Maximum cable length of 20 m
- Maximum of 15 loads/devices per bus (e.g., one board and up to 14 instruments), with at least two-thirds of the devices powered on
Even though the maximum cable length between GPIB devices is 4m, you can still use an 8 meters cable to communicate with your devices. However, you will not achieve fast transfer rates, and your system will not comply with the IEEE specifications. If you do not need high data transfer rates, and you want greater flexibility in positioning your devices, then the 8-meter cable should work for you.
If you must exceed these limits, you can purchase bus extenders and expanders from NI.