IBM 3745 Series, Hardware Maintenance Reference

Page: 261 / 712

– When the HSS transfers data to the NCP buffers, via the DMA bus, it uses the FESH hardware.
Ÿ
One PIO per SDLC frame for transmit
Ÿ One or several PIOs per SDLC frame are possible for receive
Ÿ IOC cycle steal is used for command parameters and status, IML, dump, and SIT trace data.
Ÿ Frame Relay duplex protocol attached as a DTE to a Frame Relay network, or as part of the Frame
Relay network as a DCE.
Ÿ
Multiple Frame Relay frames with one transmit PIO.
Ÿ One PIO per received Frame Relay frame.
Ÿ Use of a SNAP header format table to place Frame Relay headers and data in the proper alignment
for NCP in their receive buffers.
Main Differences with LSS
Ÿ
Buffering of the receive and transmit frames is done in a FESH buffer instead of CSP storage.
Ÿ
SDLC transmit continue command not supported.
Ÿ SDLC receive monitor command not supported.
Ÿ No multiplexing (only one line).
Ÿ
HSS microcode deals with buffer prefixes only with scanner interface trace (SIT). The remaining
buffer prefix handling is done by the FESH.
Ÿ
CCU storage access for NCP buffer data is done through DMA instead of IOC cycle steals as in LSS.
However, IOC is still used for the following:
1. Dump, IML, diagnostics.
2. Sending status (followed by CCU level 2 interrupt) when FESH, DMA or SCTL error has to be
reported.
3. Facilities (Refer to “Problem Determination Aid” on page 5-37) invoked from the operator console
(MOSS) to:
a. Perform diagnostics.
b. Access FESH registers and random access memory.
4. IOH transfer between NCP and CSP.
5. MOSS exchange with HSS.
Chapter 5. High Performance Transmission Subsystem (HPTSS)
5-9