Pinball Wizard

Standing in Joe's office on the upper floor of the Greenfield Avenue building, we were doing our honest best to discuss the problem at hand while Scott Zifferer clacked and banged away on the pinball machine in front of Joe's desk.

The network that Allen-Bradley chose for their new SLC-500 controllers was DH-485, a simple token-passing network, usually running at 19.2 kilobaud. On the bright side, very simple, inexpensive off-the-shelf hardware could be used to interface to the network. Not so great was the computing power of late 1980s personal computers and their rather pathetic serial hardware (no interrupt on transmitter shift register empty? Aaaahg!). These machines became rather bogged down with the constant barrage of communication interrupts, and the poor things spent a large part of their time running our state machine code just to stay on the network. Some slower systems preferred not to run at all.

The problem Joe and I were discussing that afternoon was how we might relieve the PC of some of the communications burden. Now, I don't believe Allen-Bradley had come out with their KR card yet, though my memory is uncertain of this. But the KR was a weird beast in its own right. I rather liked the Zilog Super8 microcontroller at its core, and had fun writing a Super8 disassembler to do our reverse engineering. But the crazy thing had a dual-port memory 32K wide, which was a huge chunk of upper memory on a pre-1990 PC. Asking customers to install an expensive KR card and give up a big chunk of memory to use DH-485 was unappealing.

Much as we hated dabbling in hardware (too much headache for too little return), Joe and I were discussing the possibility of doing our own DH-485 coprocessor card. Zifferer drained another ball in the pinball machine and quipped, "Can't you just use the KT card?"

Joe and I looked at each other; that was it. Scott grinned when he saw that he'd shut us both up so effectively. ICOM's DH-485 SmartCable was born.

The KT card was the ISA followup to the KL, which had been a two board design for the proprietary bus of a re-branded Data General One computer. Designed to handle Data Highway and Data Highway Plus networks, it featured a Zilog Z80 CPU, SIO, and 64K of RAM, 4K of which was shared with the host computer. The coolest thing about the card was that, unlike previous communication adapters, new software could be loaded onto it. We could offload the DH-485 state machine to the KT or KL, and use the 4K dualport memory as a mailbox for incoming and outgoing packets. Eventually, when Allen-Bradley would come out with their PCMCIA version of the card, the PCMK, we'd port our driver to that as well, and issue a new SmartCable to match.

The SmartCables were enormously popular with users. No longer did they need separate cards for these different Allen-Bradley PLC networks. The DH-485 SmartCable was a true ICOM innovation, the sort which earned us many friends through the years.

Never underestimate the value of having a pinball machine in one's office.