PDP-11 front-end for a VAX-11/780

An unpublicized feature of the VAX-11/780 is the provision for attaching a PDP-11 to the VAX UNIBUS Adapter. Doing this can give significantly improved I/O performance for applications which are limited by overhead in the VAX I/O driver rather than by the transfer speed of the UNIBUS itself. Such a system was implemented by using a PDP-11/04 as a front-end to a CAMAC data acquisition system. Both the PDP and the VAX have full access to the UNIBUS. That portion of the PDP address space that does not have UNIBUS memory can be mapped to buffers in the VAX memory; this approach allows the PDP to access VAX memory and to initiate DMA transfers directly to the VAX. The VAX also has full access to the PDP memory; a convenient means for developing and downloading the PDP software is thus provided. 5 figures