the length of stub cable should not exceed 1-foot, but again this may be exceeded if installation requirements dictate. The isolation resistors R have to have a fixed value of 55 ohms ± 2.0 percent. Bus Couplers Stubs for RTs, the BC, or BMs, are generally connected to the bus through coupling boxes, which may provide a single or multiple stub connections. These provide the required shielding (≥ 75 percent) and, for transformer coupled stubs, contain the coupling transformers and isolation resistors. They have two external connectors through which the bus feeds, and one or more external connectors to which the stub or stubs connect. These stub connectors should not be terminated with matching resistors, but left open circuit when not used, with blanking caps where necessary. One of the bus connectors may be terminated where the bus coupler is physically at the end of the bus cable, i.e. it is not normally considered essential to have a length of bus cable between the last bus coupler and the termination resistor. Cable Termination Both ends of the bus, whether it includes one coupler or a series of couplers connected together, must be terminated (in accordance with MIL-STD-1553B) with "a resistance, equal to the selected cable nominal characteristic impedance (Zo) ± 2.0 percent." This is typically 78 ohms. The purpose of electrical termination is to minimize the effects of signal reflections that can cause waveform distortion. If terminations are not used, the communications signal can be compromised causing disruption or intermittent communications failures. Connectors The standard does not specify the connector types or how they should be wired, other than shielding requirements, etc. In lab environments concentric twinax bayonet style connectors are commonly used. These connectors are available in standard (BNC size), miniature and sub-miniature sizes. In military aircraft implementations, MIL-DTL-5015 and MIL-DTL-38999 circular connectors are generally used. Similar systems DIGIBUS (or Digibus, GAM-T-101) is the French counterpart to MIL-STD-1553. It is similar to MIL-STD-1553 in the same notion of Bus Controller, Remote Terminal, monitor, same transmission speed, but the difference is that DIGIBUS uses separate links for data and commands. GOST 26765.52-87 and its descendant GOST R 52070-2003 are the Soviet and Russian, respectively, equivalents of MIL-STD-1553B. The encoding, data rate, word structure, and control commands are fully identical. GJV289A is the Chinese equivalent of MIL-STD-1553. Aircraft utilizing this system can reportedly use both Soviet (GOST bus) and western (MIL-STD-1553