The Expandabus

written by Matthew Reed

The TRS-80 Model I, unlike some other contemporary computers, offered no way to install expansion cards internally. The Model I did have an expansion bus connector which allowed users to attach external hardware devices, such as the Radio Shack Expansion Interface. But the single connector meant that only one hardware device could be attached to a Model I at a time.

The Expandabus from Alpha Products, introduced in 1981, provided a way for TRS-80 Model I owners to connect multiple devices to the expansion bus, allowing up to five devices to be connected to a single Model I.

The Expandabus was similar to the T-Buss, which was one of the first TRS-80 items Alpha Products sold. The T-Buss cost $80.00 and allowed five devices to be connected to the Model I. The Expandabus was more flexible and available in four versions:

• the X2-40 for $29.00, which could connect two devices.
• the X3-40 for $44.00, which could connect three devices.
• the X4-40 for $59.00, which could connect four devices.
• the X5-40 for $74.00, which could connect five devices.

The Expandabus connected directly to the expansion bus connector on the Model I, or to the expansion connector (also known as the Screen Printer port) on the Radio Shack Expansion Interface. All connectors on the Expandabus were gold-plated for maximum reliability.

In mid-1984, Alpha Products stopped using the Expandabus name in their advertisements and began calling it the “Y-Cable for Model I.” They also added a new product: the “Y-Cable for Model III and 4” which was essentially an Expandabus for the TRS-80 Model III and Model 4. The “Y-Cable for Model III and 4” was available in three versions:

• the X2-50 for $34, which could connect two devices.
• the X3-50 for $49, which could connect three devices.
• the X4-50 for $64, which could connect four devices.

Compatibility

Not all hardware for the TRS-80 would work with the Expandabus. There were several potential problems:

• Some devices didn’t completely decode the TRS-80 addressing signals because they assumed that they would be alone on the expansion bus.
• Some devices exhibited timing conflicts when they had to coexist with other devices.
• Some devices shared the same port addresses and couldn’t be used at the same time.

But most TRS-80 devices did work. Not surprisingly, hardware sold by Alpha Products, such as the Alpha Joystick and the VS-100 Voice Synthesizer, worked fine with the Expandabus.

Unlike a somewhat similar device, the Radio Shack Multi-Pak Interface for the TRS-80 Color Computer, the devices plugged into the Expandabus weren’t switched but were all available to the TRS-80 at the same time.

To avoid any possible conflicts, most TRS-80 hard drive manufacturers recommended that the Expandabus not be used with their hard drives. I know of many people who used it anyway and never experienced any problems.

Alpha Products sold the Expandabus/Y-Cable as late as 1990.