Litton Industries Monrobot XI computers available

The big part of the difference in speed between the PDP-8 and the Monrobot XI is that the PDP-8 used random-access magnetic core memory as the main memory and a parallel architecture, and the Monrobot machines used magnetic drum and a bit-serial architecture. In drum-based architectures, all operations, including instruction fetch, operand fetch, and (not sure with the Monrobot XI, but some machines did have all internal registers also located on the drum) register access had to wait for the drum to rotate to the proper location in order for the data to be read. In many cases, the master timing for the machine was generated by a dedicated pre-written clock track on the drum. Thus, the speed of the machine was limited by the rate of rotation of the drum and the density of the clock track. Along with all of the waiting time for the drum, the logic of the machines of this ilk was serial, which mated well with the fact that the data comes streaming off the drum in bit-serial form (in most cases). Serial logic is inexpensive...but slow (observe the performance differences between a PDP-8/L and a PDP-8/S). Another key difference between the early PDP computers and devices like the Monrobot machines is that the PDP was designed as a general-purpose computer, with expandability and a generalized I/O architecture to support a wide range of interfaces. Machines such as the Monrobot series were quite fixed in their architecture. There was no way to add more memory, or interface to more than a few external devices. The architecture was quite fixed. The Monrobot series of machines started out as early desk-sized sequence-controlled electronic calculators, with decimal arithmetic, and math-oriented functions. These machines were originally designed for automating accounting and bookkeeping operations. Some made their way into engineering and scientific persuits, though. The machines had little in the way of bit manipulation functions. As time and the marketplace went on, the Monroe machines (and others of the time, such as those made by Smith Corona Marchant (SCM), and Friden) began to take on more computer-like functionality...switching from decimal-based arithmetic to true binary, adding boolean logic functions, and providing more sophisticated branch and looping functionality...moving the machines from the realm of sequence-controlled calculators to true computer capabilities. Rick Bensene The Old Calculator Web Museum http://www.geocities.com/oldcalculators