ZCLK, First "Big" Product

The ZCLK is a calendar/clock module for the Zenith Z-100 series computer. It uses a trick so as to avoid using one of the machine's S-100 expansion card slots. To install the ZCLK you unplug a chip off the motherboard and plug it into the ZCLK then you plug the ZCLK into where the chip was on the motherboard. The particular chip involved provides, among other functions, the parallel printer port for the Z-100. The ZCLK cleverly shares the port without interfering with normal printer operation. The idea was novel enough to receive U.S. Patent No. 4,706,189, Special Purpose Peripheral Circuit And Method Of Connecting The Same To A Computer. The sharing of printer ports by non-printer devices is still a popular technique. Starting with the Spring/Summer 1985 issue, the ZCLK was available through the Heathkit catalog for a year. During this period, it could also be purchased at Heathkit Electronic Centers, nationwide.

ZMF100, The Big Seller

The motherboard of the early H/Z-100 computers used 64k RAM chips. The ZMF100 allowed this motherboard to use 256k RAM chips. The ZMF100 worked much like the ZCLK except that you removed two chips and plugged in two small circuit boards which connected together with a cable. You also had to remove the motherboard from the computer and install a wire (by soldering) connecting the pin-1's of the 27 memory chip sockets together and to a little cable which connected to one of the small circuit boards. This wasn't much of a problem in those days as many owners had built their own "Z-100" from the Heathkit version and were skilled at soldering. (Even so, the motherboard in the kit was supplied pre-assembled.) The ZMF100A revision (pictured) simplified things. There was now a single circuit board and a bus bar assembly (just visible at the lower left in the picture) that plugged into the memory chip sockets to connect the pin-1's. No soldering required.