IBM Midrange Connectivity

 


IBM's Midrange systems (AS/400, Advanced System 36, System 34, System 36, System 38) are popular workhorses in business applications worldwide, with over 250,000 units sold. These systems were designed to use a host-terminal connectivity scheme, with almost all processing being done in the host system, and terminals which merely display data generated by the host. They were designed to provide nearly instantaneous screen updates to give the appearance of a fast system to the user.

In order to achieve the goal of fast updates, IBM designed a proprietary interconnection standard called 5250, which provides a 1 Mb/s transfer rate from host to terminal. The 5250 system originally used only twinaxial cable, but it has been adapted over time to also support unshielded twisted pair (UTP), the IBM Cabling System, and fiber optic cable. There are also methods provided to allow remote connectivity to the system.


Basic Twinax Connectivity

The original, and conceptually simplest, 5250 connectivity solution uses twinax cable in a bus (daisy-chain) topology. In this solution, a twinaxial cable is run from the host to the first terminal, then another cable is run from the first terminal to the second terminal. The process is repeated until a maximum of seven (7) terminals is connected over a distance up to 5000 feet from the host. Figure One shows this type of setup.

Figure One
Twinax Daisy-Chain


Advantages & Disadvantages

Advantages

Conceptually Simple
The daisy chain is very simple and easy to understand. It can be very easy to plan an initial installation of this type of system.
 
Relatively Inexpensive
Since there is no equipment other than cable used for this type of setup, it can be the least costly way to connect terminals to the host system.
 
Long Distances Possible
Twinax cable allows a segment to be up to 5000 feet in length from the host's port to the last terminal.
 

Disadvantages

Difficult Moves & Changes
Moving a terminal requires cutting and/or splicing the twinax bus. This type of system is not a good choice if there is any anticipation of a need to change the setup after the initial installation.
 
Reliability & Troubleshooting
The general nature of a bus topology almost guarantees that a problem at any point in the bus will drop the entire segment. It is also very difficult to determine where the problem is. Therefore, when a problem happens, up to seven users may be down while a long troubleshooting process is started.
 
Ground Loops
Since the twinax cable's shield is connected to ground at the host and at each terminal, small changes in the ground potential at each device can result in a noise-inducing electrical current being induced into the shield of the cable. This is very difficult to troubleshoot and sometimes impossible to resolve. It also may be intermittant in nature.
 
Proprietary Cable
Twinax cable is useless for any other technology. Therefore, if an organization decides some time in the future to change from a host system and terminals to PCs and a LAN, then the entire investment in twinaxial cable would have to be scrapped and the building rewired to support the new system.

Overall, twinax-based systems tend to have more disadvantages than advantages. In order to avoid these problems, UTP cable setups have emerged.


UTP Based 5250 Networks

Unshielded Twisted Pair cable is a very popular medium for all network types, and it is commonly used for IBM Midrange systems. Properly designed, a UTP cabling plant, also called a Structured Cabling System will support any network type without changes.

UTP cable is always run in a star topology, with each terminal having its own cable run to a common hub. Figure Two shows a basic UTP setup.

Figure Two
Basic UTP 5250 Network


UTP Network Structure

Figure Two, above, illustrates a typical UTP based 5250 network. In that diagram we have illustrated several terminals connected to a single controller port via an active hub. We will now explain how the system is interconncected.

The signal originates from a twinax port on the Controller. On that port, we have installed a twinax balun, which converts the physical interface from a female twinax connector to an RJ-45 UTP connector. A short patch cable then connects the balun's RJ-45 connector to a host port on the hub. The hub then amplifies, splits, and retimes the signal, then sends it to the individual terminals via UTP cable runs. Each UTP cable run then connects to a twinax to RJ-45 balun which is in turn connected to a twinax "T" connector which then connects to the terminal.

NOTE: Unless the balun is specifically marked as being "Terminated," do not omit the twinax "T" adapter! The "T" adpater provides a 100 ohm termination which is critical to the functionality of the system. Omiting this termination will make that terminal unreliable at best, and will most likely result in it not working at all.


Advantages & Disadvantages

There are pros and cons to any technology, and 5250 over UTP cabling is no exception. Below are the advantages and disadvantages to using this technology.

Advantages

Cabling Reuse
A UTP based 5250 network will normally use a structured cabling system, which is useful for other applications. Therefore, changing from a midrange environment to a LAN does not require a wholesale swap of the cable installed in a building.
 
Flexibility
Structured cabling systems are, by their nature, flexible. This makes it easy to move terminals after the initial installation.
 
Reliable & Easy To Troubleshoot
Because each terminal is isolated from the other terminals by the active hub, a fault on one terminal is unlikely to affect any other terminal. This allows a quick determination of the source of any problems, and allows faster and easier troubleshooting than a twinax bus allows.

Disadvantages

Higher Initial Cost
Because of the need for an active hub and baluns, the initial installation cost may be higher than the cost of a twinax-based system. Note, however, that over time the cost in effort of maintaining and repairing a twinax-based system may well eliminate the initial cost savings of that system, and actually end up costing more in the long run, especially if lost productivity due to outages is considered.
 
Single Point Of Failure
A failure of a hub, although fairly uncommon, will result in all terminals attached to it to lose connectivity to the host system. It is generally a good idea to keep a spare hub on hand for such emergencies.

UTP Caveats

Baluns

It is worth mentioning that there is no industry-wide standard type of balun. Each manufacturer has its own standard regarding the active pins and polarity of their hubs and baluns. Therefore it is best to avoid mixing hubs from one manufacturer with baluns from another. They can be made to work with each other, but doing so requires experimentation in the wiring of the patch cables to match a particular signal on one pin of the hub's connector with the corresponding signal on the balun, and they will most likely be different.

UTP Daisy Chains

Occasionally, attempts are made to eliminate the hub from the system, and interconnnect the host and terminals by daisy-chaining baluns together with twisted pair cable. This is a very bad idea for several reasons. The first reason is that the baluns have a very high signal loss compared with twinax connectors, which results in a system which will only support 2 - 3 terminals (at best). A second reason is that, even if it is made to work, by its nature such a system is generally unreliable, and just as difficult to troubleshoot as a twinax bus. The end result is that we end up with a network which is inferior to either a twinax bus or a normal hub-based UTP network. For these reasons any attempt to daisy-chain devices together with baluns and UTP cable is very strongly discouraged.

Passive Hubs

Nowadays, it is not reccomended to use passive (non-powered) hubs for UTP-based 5250 networks. These units are merely signal splitters which do not provide any type of isolation between the terminals. Therefore, any fault at any point will most likely result in all terminals on a particular controller port going down, and troubleshooting which is just as difficult as a twinax bus network. Also, these units do not work well with the AS/400 or Advanced System 36, as these units have a far weaker signal and tighter timing requirements than the older Systems 34, 36, and 38. When used with the newer systems, passive hubs generally can only support 2 - 3 terminals at very short distances, and adding more will usually drop the everything attached to the controller port the hub is attempting to split.


IBM Cabling System (STP)

A third type of midrange connectivity solution is to use the IBM Cabling System which is commonly used for Token Ring networks. This system uses special 150-ohm shielded twisted pair (STP) cabling to create a physical star and logical bus topology. Such a network is illustrated in Figure Three

Figure Three
5250 Over The IBM Cabling System


Description

Figure Three illustrates a 5250 network which is built with the IBM Cabling System. The signal originates from a twinax port in the Controller, and adapted to the 150 ohm STP cable via a device called a Green Balun. The signal then goes to the first port of a device called an MCL Panel. From the MCL Panel, the signal then goes to an IBM Type 1 cable to a wallplate.

At the wallplate, all terminals except the last one use what is known as a Y Cable Assembly. This cable has an IBM Data Connector at one end and two twinax connectors at the other end. The twinax connectors then connect to the terminal through a twinax "T" adapter.

The last terminal in the chain connects in a different manner from the previous six terminals. This unit is connected with a Twinax Direct Cable, which has a Data Connector at one end and only a single twinax connector at the other end. The twinax connector connects to one side of the last terminal's twinax "T" adapter. The other end of the "T" adapter then must have a 150 ohm Green Terminator attached to it to terminate the line properly.

In actual operation, this system takes advantage of the fact that the Type 1 cable has two pairs of wires in it. Starting at the controller, our signal enters a Green Balun, which converts its impedance and sends it to the first pair in the Green Balun's Type 1 cable. This pair then enters the MCL Panel's first port. The signal is then routed to the first pair of the second port's connector, and is routed into the first pair of the cable attached to that port. The signal then reaches the Y Cable for the first terminal, loops up to the first twinax connector, to the terminal, out the second twinax connector, and goes back up the second pair of the Y cable. When it again reaches the wallplate, the signal then travels back through the second pair in the Type 1 cable, reenters the MCL Panel's port, and is looped over to the first pair of the third port of the MCL Panel. this process repeats itself until the signal arrives at the terminator.

System reliability is enhanced by an automatic loopback function built into all of the Data Connectors used in the system. If a cable is disconnected, the signal will be immediately shunted at the point of the break to the second pair of that connector. Therefore, disconnecting a Data Connector will have no effect on the rest of the system.

This system is a good choice whenever a building is already wired with the IBM Cabling System, or there is a need for good shielding. Otherwise, UTP or Twinax-based systems are a better choice. The IBM Cabling System is basically only useful for 5250 connectivity or IBM Token Ring. Therefore, if this system is installed, and the organization wants to go to a LAN thereafter, the cabling plant will have to be scrapped and reinstalled unless the LAN is a Token Ring. For these reasons, this type of installation is falling in popularity in favor of UTP based 5250 networks.