Robotron A5120

The predecessor of the PC1715 is the A5120. It is also equipped with the U880 processor. It even has two processors. However, this is not a true dual-processor system; rather, the two CPUs are connected in parallel. Only one of them is active at any given time. Upon an interrupt, the system switches to the second processor, enabling a very short response time because the CPU registers do not need to be saved. Another feature is memory protection: The CPU can protect individual 64-bit areas of its memory from being overwritten. Similarly, user access to the I/O ports can be intercepted.

Unlike the PC1715, the A5120 is not yet built on a single circuit board. Instead, the CPU board, memory board, video board, and interface board are plugged into a backplane. However, the floppy controller of the PC1715 and the A5120 are very similar in terms of the type and location of the microcontrollers used.

Screen with CP/A startup message

A whole range of very different operating systems were available for the A5120. Besides the well-known ones like SCP and CP/A (both CP/M derivatives) - here's an image showing the messages the operating system displays during startup - there's, for example, UDOS, which is practically a 1:1 copy of Zilog's RIOS operating system, so much so that even the Zilog copyright notices are still included in the binaries.