Circuit Rider
Version 3.10
By Jim Scheef

Value

One of my many computer obsessions involves collecting IBM PCjr “stuff.. Collecting things is part of human nature, especially for the male of the species. My ski collection may be more related to my inability to throw things away than a true collection, but that’s another story. The PCjr collection began as part of a collection of early IBM microprocessor-based computers (5110 Model 3, 5150, 5150r2, 5160, and the 4860 – the PCjr. From here the collection expanded to DOS non-compatibles like the Zenith Z-100 All-In-One, several early incompatible laptops, and my prize, a Seattle Gazelle (made by the company that sold DOS to Microsoft). To get back on the track that is not yet apparent, my PCjr stuff includes as many of the cartridge-based programs that I could find over the years, like the original King’s Quest from Sierra Online. Just plug in the cartridge and start exploring—no DOS needed! Many games were sold this way, just like games for Atari home computers. IBM produced both games and educational programs on cartridges.

All of this leads to my current quest—to network the IBM PCjr. Back in the early 1980’s, IBM produced and presumably sold two local area networks – the IBM PC Network and the IBM Cluster. The PC Network was a broadband network intended for businesses, while the Cluster was intended for “the education market.” The Cluster was the “network” that included the PCjr. Actually IBM did not use that word in relation to the Cluster, so as not to confuse the business market into thinking that the PCjr was suitable for office use. As you might imagine—had you ever given it an iota of thought—documentation on these networks is just about impossible to find. So when I saw the book, Networking with the IBM Network and Cluster, by Michael Hurwicz, on eBay, I bought it in a heartbeat. After skimming through this book, I now understand why it took companies so long to implement local area networks. Today, we take LANs for granted. It’s now the norm for even very small offices to have all of their computers on a network that allows them to share printers, disk storage and that all-important connection to the Internet. When was the last time you sat down to use a computer that was NOT connected to the Internet?

The IBM PC Network was a broadband network. What does that mean? The main thing it means is that the network itself—the hardware part—was incredibly complicated to set up. A broadband network works very much like your cable company’s network in that different radio frequencies are used to carry different things. Like cable, a broadband network can carry video, and that was one of the selling points. A company could get more value from the cost of installing a lot of coaxial cable in the walls of their offices. Remember, we are talking about networking the original PC with 640K of RAM – max! If you think that a simpler computer made life easier, you have a short memory. One of the limitations of the IBM PC Network was that each ‘network’ could only have eight computers connected to a splitter (the pictures look similar to the splitters we use today to connect several televisions to the cable system - but bigger, of course). Something called frequency translators were needed to connect multiple network segments and attenuators and directional couplers sat in the middle. The actual data was carried in packets (like today) but the bits were encoded by modulating a radio frequency carrier wave – much like a radio station. Thank God, we moved on.

The IBM Cluster used simpler technology called “baseband.” Once again, the physical media was coaxial cable but the information was sent using a signaling technique much like ethernet over coax. Each computer was connected to the cable using a “tee.” The cable went from computer to computer with a terminator on the unused side of the last “tee” at each end of the chain. The Cluster adapter card in each computer had a set of “DIP” switches that were used to set the unique address of each computer.

Both types of networks required software on each computer. In the Cluster, much of this software was in a ROM on adapter card. The Cluster allowed for one master computer and several slaves. The intention was that most of the slaves would be PCjr's used by students while the teacher used the master computer, an IBM XT, of course, to control the class. The slave computers did not need a hard disk because they pulled any software from the XT. Twenty-five kids simultaneously loading the same program from an XT must have been excruciatingly slow! The only plus was that programs back then were "a little" smaller than today. The Cluster had a data transmission speed rated at 375Kbps (thousand bits per second). The PC Network ran at 2Mbps. In this same era, 3Com's Ethernet ran at 10Mbps but was regarded to have a throughput closer to 3Mbps. Compare these to the 100Mbps we consider normal today in our home networks and the 1Gbps and 10Gbps networks that are becoming more common in corporations.

OK, we're getting to the point of all this. How much did all this cost? Well, the table above gives some prices.

Look at how prices have changed in 20 years and how much more you get for your money. I have no idea how much one of the "frequency translators" or a signal splitter cost, but I can guarantee that they cost much more than you paid for the router that right this minute is guarding your connection to the Internet. Today's most common network speed of 100Mbps is more than enough to stream video around the house. Soon we will have wireless networks that actually run even faster!

Other stuff
Last month I promised to talk about how to get porn for your Video iPod. I skipped it because I'm sure you figured it out on your own. Next month I plan to talk about RFID and why the DHS is looking for RFID scanners that can read a tag in a moving car. Stay tuned.


 
 
© Danbury Area Computer Society, Inc. All Rights Reserved.
Web Site Terms & Conditions of Use