Interfacing With the RB Series Accessory Connector
Last Revision: February 18, 2004

The information in this article applies to:

• Users of a Cedrus an RB-530, RB-730, RB-830, and RB-834 response pad who want to connect their own hardware to the Accessory Connector found on these products. If using an earlier model, click here.

Summary

The RB-x30 response pads include an Accessory Connector that offers up to six I/O (input/output) lines. These I/O lines can be used to connect external peripherals such as additional keys or a foot pedal, or to obtain a TTL output of the built-in keys. This document describes the software and electrical features of the Accessory Connector.

Features

The Accessory Connector uses an RJ45 jack. This is the same type of jacks used in Ethernet wiring, making it easier for find and buy matching cables in a wide variety of lengths and colors.

The RJ45 jack has eight pins. One is ground. Another is positive voltage. The remaining six are I/O lines. The table below describes the pin assignments. Pin 1 is the leftmost pin when you are looking at the connector.
 

When in Reflective mode (see the description of the Accessory Connector for information about the difference between Reflective mode and General Purpose mode), a key on the response pad corresponds to the following Accessory Connector pins:
 

Building Your Own Switch

The RB-x30 response pads use an "active low" logic. In other words, when the voltage on an I/O pin is around 5 volts, the line is considered OFF. When the voltage is near zero, it is considered ON. Therefore, your own switch should connect between the desired I/O line and ground (pin 8). An active low approach may seem upside down to some, but it is a common practice in electronics.

Debouncing

If you are planning on connecting a switch, it is important that you become aware of debouncing issues.

What is it?  When a key is pressed once on the keyboard, we take it for granted that one and only one letter is typed. In reality, closing a switch causes the two metal contacts to touch each other several times over a period of time. This is called bouncing.

Debouncing is a method used to recognize the first switch closure but ignore the subsequent bounces. This can be done using either hardware or software. The RB-x30 response pads have a built-in microprocessor, so debouncing is implemented using software. The basic software method is to simply ignore subsequent bounces for a certain amount of time. There is no delay between the key press and the information about it being sent to the computer.

In the RB-x30 models, the timeout duration can be set by the user. It is applied intelligently. For example, if you set the debouncing duration to 5 milliseconds and the user presses a switch at time T, then all subsequent input on this switch is ignored until time T+5 milliseconds, but meanwhile input on all the other switches is still detected even at time T+1.

If you are doing your own programming, see the XID Set Debounce Time command.