Expressway intelligence can interrogate the plotter and send setup info to the computer program if required.
The Expressway can be set-up for "listen only" use which allows the unit to be used with instruments which require peripherals to be set to "listen only."
The Expressway accepts data at up to 20K Bytes/second and send it to peripherals at whatever rate each device can accept.
Check with Eventide for benchmark data on productivity improvements obtainable with the Expressway and various combinations of HP hardware and software. See the back page for typical examples. We can confirm Expressway compatibility with a wide variety of hardware/software configurations. Call us at (201) 641-1200.
Inherent in the operation of the IEEE-488 bus is the concept of the "handshake." Every time a byte of data is sent out from the computer, a handshake line is also asserted. When the byte is accepted by the peripheral, the handshake is completed and the computer is allowed to send another byte. If the peripheral is not ready for another byte, the handshake is not completed, and the computer is prevented from continuing. (this explanation ignores the "three wire handshake" which gives the bus many of its many unique capabilities, but is irrelevant here.)
If the peripheral doesn't handshake, the simple-minded response of the computer is usually to wait until it does. If the printer can accept, for example, one line of data at a time, and hold up further data while that line is being printed, data will be transferred in bursts with "dead time" of some milliseconds between bursts.
The further limits throughput speed because only one set of cables is used to transfer all information to all peripherals. It is impossible to take advantage of the period of time between data bursts when a peripheral is holding up the handshake to send data to another peripheral. Thus is we have data to send to the printer, and a graph to plot on a plotter, we must do these tasks sequentially. If each takes fifteen minutes, both tasks will take half an hour. During this half hour, the computer's human operator is wasting time. And the computer is using its vast powers to just monitor a handshake line.
Expressway Improvement Benchmarks - Let's consider how the Expressway can eliminate all this waiting. Typical printers operate in the range from about 30 to 300 characters per second. The Expressway can accept data at up to 20K Bytes per second, so it all goes according to the oversimplified explanation above, this implies an improvement of 66 to 666 times. Plotters are not so simple to characterize. Factors such as pen acceleration and ultimate speed are basic considerations, but you determine the line length and ultimate complexity of the plot. Also, in many applications, pen velocity is restricted due to pen and media considerations. It's entirely possible to have conditions in which the plotter can accept an average of only 5 to 10 Bytes per second. In such cases, performance improvements on the order of several thousand can be obtained with the Eventide Expressway.
The first key to the performance improvement is the buffer's storage capability. With up to a megabyte of memory, the Expressway can store hundreds of pages of information, or very large amounts of data to be plotted. Usually the computer will be able to send all its data to the Expressway in one burst, at its maximum rate. The second key is the two port output of the buffer. Because the unit has two independent sets of cables, the "one peripheral at a time" limitation discussed above, does not occur. Even if one peripheral is refusing data, the second can be receiving it.
Of course your actual performance gains will depend on the nature of your output, and the specific characteristics of your peripherals. The first time you have a large area to fill with a thin pen in your plotter, you'll bless the Expressway. In other circumstances of course, productivity gains would be less. It all adds up to this: With the Eventide Expressway, you can use your computer printer and plotter simultaneously and full-time.
What's inside - The Expressway is a computer in its own right. It uses a Motorola 6809 pseudo-16 bit processor as its CPU, and has its internal operating program stored in an EPROM. The unit comes with 256K Bytes or one megabyte of RAM, of which 16K is reserved for the CPU. The balance is used to store two data queues. Each is tagged as to which peripheral it belongs. In this way, memory is divided efficiently between the two ports regardless of how much data is available for each peripheral. The computer is connected to the PORT 0 circuit board, and the peripherals are connected to ports 1 and 2. All port boards are physically interchangeable and are configured by software.
When power is applied to the Expressway, the CPU reads the address switches and configures PORT 0 to be a LISTENER and TALKER on these addresses. It also configures PORTS 1 and 2 to be CONTROLLERS, so they can configure the bus and send data messages to the peripherals. The effect is to make the computer believe (with limitations) that it is still connected to peripherals at their respective addresses. (Additional peripherals connected to the computer are not affected and operate normally.) If either PORT 1 or PORT 2 is configured to be connected to a plotter, the Expressway, acting as CONTROLLER, interrogates the plotter, asking questions such as model number and capabilities. These are relayed to the computer upon request. There are no operating controls, but the front panel has 8 status LEDs which confirm results of the unit's power-on self-test routine. In operation, the LEDs indicate activity status at each port.
Extending the bus - The Expressway can also be used to extend the IEEE-488 bus in three ways. Although the bus protocols allow for the connection of up to 30 peripherals, physical limitations hold the number down to 15 units. The Expressway appears to the bus as a single peripheral, yet has two output ports, enabling you to add an additional peripheral to an otherwise full bus. Further, you can add more than one peripheral to each Expressway output port (actually up to 15) if the peripherals are set to the same address. Lastly, the Expressway can physically extend the nominal 20 meter maximum length of the bus by an additional 20 meters from each output port.
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