Asynchronous Devices on the VS Get in sync with async options
From "VS Workshop",  Access 88, February 1988
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This month I'll complete my review of peripheral connection options with a discussion of the asynchronous options available for VS users.

As we covered last month, most Wang peripherals are connected to the VS using a proprietary synchronous interface generally known as the 928 connection. The signal is sent at high speed through dual coaxial cables for distances of up to 2000 feet. By contrast, asynchronous devices communicate at a slower rate through copper wire to a standard connector for distances of up to 50 feet (maybe a bit more), using four or more signal wires.

Advantages of asynchronous devices lie in their flexibility of configuration and support across manufacturers. For example, it may be necessary to use an async approach to connect specialized workstations, printers, or plotters that Wang does not offer. It is also possible to use the async approach to gain access to the VS from a non-VS system.

Another advantage is the independence of the method used to get the signal to the VS. It makes little difference whether the device is connected locally or communicating from a distance, using a modem. This means that the user can have virtually the same access to the system whether remotely or locally connected.

Price is one of the oft-quoted advantages of async applications - though the larger cost of controllers can negate this advantage. A careful analysis of ALL costs related to installation should be completed before async terminals or printers are selected.

Disadvantages of asynchronous connections include the speed and reliability of the transmission. Speed is hindered by the need to transmit non-data elements (known as start and stop bits) to define the beginning and end of a character. Since more bits are needed to transmit the same data, the transmission is effectively slowed. Similarly, minor errors are more likely to occur, requiring retransmission of the data. Modern error detection and correction techniques and better data compression algorithms have made both of these issues less important.

Another typical disadvantage of asynchronous communications is the maximum cable distance that can be supported without amplification. Wang uses a variation of the RS232 standard for their async communications, which is normally limited to cable distances of 50 feet. However, Wang claims that cable distances of up to 2000 feet are possible with the 2110 and 2110A workstations. One can only guess that the workstation is responsible for this difference. Hmmmm. . .

What's available?

Wang's asynchronous support includes the 2110 and 2110A workstations, several printers, and two major categories of controllers. Smaller VS models (the 5, 15, 6, and 65) have a spare connector on the back that is suitable for a printer and are also available with async ports instead of the usual 928 connectors. Due to the reduced circuitry involved (and marketing influences!) the prices for async workstations and printers are 40-70% of the equivalent 928 devices.

Getting data into the VS: controllers

Asynchronous devices are supported on the VS through the use of two major categories of controllers: those that support microcode and those that do not. Since it is the controller - not the terminal - that accepts microcoded instructions from the VS, the type of controller you choose depends on the applications you expect to support. Examples of programs that require microcode include VS Word Processing, Multi-Station, and some programs for communications support, such as X.25 (packet-switched networks).

If your application mix does not include WP or Multi-Station, the Asynchronous Device Controller (ADC) or Multi-Line Telecommunications Controller (MLTC) provide basic RS232 connections that can be used with a variety of data processing applications. ADC controllers come in units of 8 or 16 ports, and at present all ports must be similarly configured for speed and device type.

If your application mix includes microcode-dependent uses, the controller you need is the Enhanced Asynchronous Device Controller (EADC). This is available either in single-port or 8-port flavors, with the single-port version consisting of a complete Wang PC running special software to act as a dedicated VS controller. (Yes, you can also get the PC software by itself.) If cost is the only criteria, neither approach represents a cost-effective method of connecting peripherals; the per-workstation costs are comparable to a 4230A and a 928 port for it. By the time you read this, however, Wang should be shipping a 32-port EADC intended to offer better costs per connection to replace both of these options.

Supporting more users

If your shop is like ours, the number of simultaneous users is somewhat less that the total number of workstations you have defined to be attached. If you are also near the maximum number of workstations that can be defined for your processer type (e.g. 128 for a VS 100, 192 for the 300 or 7310), you are also painfully aware that those idle workstations are robbing you of potential users. Wouldn't it be nice it you could simply reuse the same ports for other users?

Imagine a workstation situation where every user logging onto the system receives the next available port rather than reserving a specific workstation number. In other words, the system would be betting that no more than 128 or 192 users would require SIMULTANEOUS access to the system, and thus be able to support more POTENTIAL users.

I refer to the approach described above as dynamic port assignment. It's available NOW by integrating asynchronous device ports with some type of switching mechanism that can handle the contention for ports. This switching task can often be handled handled by modern telephone switches (PBXs) or by specialized data communications switches. In either case, the following conditions must be met:

• Many users are involved with the system on a "casual" basis. That is, they log on infrequently and do not remain on the system for extended periods of time.

• VS Word Processing is not the primary application. (Since WP sets its defaults by workstation number, the different workstation number assigned every time would be confusing.)

• A large number of devices of the same class are to be supported. (For example, a switched array of 2110A workstations.)

• The switched elements do not include printers, TC lines, or any other device that might be referred to by static means, such as in programs or procedures.

As with most other computer systems, the VS expects certain devices at certain logical locations. On the VS, this is specified through the configuration editor, GENEDIT. Once the system has been started (Initial Program Load, or IPL), these relationships must be preserved. In other words, a similar device type must be attached to a given port. Note that this does NOT mean that the very same workstation or printer must be attached - only the same workstation or printer TYPE.

Conclusion

Asynchronous device support is one of the most interesting recent developments on the VS. With its ability to use printers and workstations from a variety of manufacturers, it offers device flexibility previously unheard of in the VS world. The independence of the means of getting the signal to the CPU (e.g. through direct cable, modem, line driver, PBX, or other means) offers the possibility of a single solution to many different types of peripheral connections. I look forward to deepening Wang support for this and other industry-standard approaches.

Next month we'll cover "power" workstations for the VS - including PC emulations of 928 and asynchronous terminals.

References:

Customer Software Release Notice: VS Device Support Release 1;
Publication 715-1316, 1st Edition, June 1987

Glossary

Term	Definition
928 connection	A proprietary interface used by most Wang SERIAL devices. Uses two coaxial cables and allows MICROCODE to be sent to the peripheral along with the data stream.
asynchronous device	A data transmission device in which characters are sent at random intervals, separated by additional control data.
data compression	Data processing techniques that reduce the transmission effort, typically by encoding redundant information.
error detection & correction	The name for a group of data processing techniques that detect errors in data transmissions and request retransmission from the sending device. Typically incorporated into MODEMS and STATISTICAL MULTIPLEXERS.
line driver	A signal amplifier that extends the distance a digital signal can be sent.
modem	Contraction of MOdulater/DEModulator. A communications device that converts digital signals to a series of sounds for transmission over analog lines, such as the public telephone network. Performs the conversion in either direction, depending on which device is sending.
PBX (Private Branch Exchange; also PABX)	A user-owned automatic telephone switching device that provides an interface with public telephone networks. Often used to distribute digital as well as voice data.
port	As used on the VS, a unique logical connection that allows connection of a specific peripheral device or class of devices.
RS232	An industry-standard interface between data communications devices that uses serially encoded binary data transmissions. Typically associated with use of the DB-25 connector, a D-shaped 25-pin connector available in male and female versions.
statistical multiplexor	A communications device that allows a single channel to carry information from several devices simultaneously.
X.25	A worldwide standard for data communications over public networks. Requires data to be assembled into "packets" that include addressing and source information.