Figure 2-10 CPC drawer logical diagram The buses are organized in the following configurations: The z14 ZR1 CPC drawer contains the two oscillator cards (OSCs): One primary and one  backup. If the primary OSC fails, the secondary detects the failure, takes over transparently,  and continues to provide the clock signal to the CPC. The two oscillators have Bayonet  Neill-Concelman (BNC) connectors that provide pulse per second signal (PPS) input for  synchronization to an external time source with PPS output. The SEs provide the Simple Network Time Protocol (SNTP) client function. When Server  Time Protocol (STP) is used, the time of an STP-only Coordinated Timing Network (CTN) can  be synchronized to the time that is provided by a Network Time Protocol (NTP) server. This  configuration allows time-of-day (TOD) synchronization in a heterogeneous platform  environment and throughout the LPARs running on the CPC. The accuracy of an STP-only CTN is improved by using an NTP server with the PPS output  signal as the External Time Source (ETS). NTP server devices with PPS output are available  from several vendors that offer network timing solutions. A cable connection from the PPS  port on the OSC to the PPS output of the NTP server is required when z14 ZR1 uses STP  and is configured in an STP-only CTN that uses NTP with PPS as the external time source.  GLYPH<SM590000> The PCIe I/O buses provide connectivity for PCIe fanouts and can sustain up to 16 GBps  data traffic per bus direction. GLYPH<SM590000> The X-bus provides interconnects between SC SCM to PU SCM and PU SCMs to each  other, in the same node. GLYPH<SM590000> Processor support interfaces (PSIs) are used to communicate with FSP cards for system  control. GLYPH<SM590000> Configurations with four PU SCMs operate as two Logical PU clusters with two PU SCMs  per logical cluster.  GLYPH<SM590000> Configurations with two PU SCMs or one PU SCM operate in one logical PU cluster. 2.3.1 Oscillator cards Chapter 2. Central processor complex hardware components  31