In response to customers’ feedback, the RB-x20 response pads now include an Accessory Connector that offers up to six limited functionality 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. They cannot be used for general purpose output. This document describes the software and electrical features of Accessory Connector.

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.

All I/O pins are pulled high via a 100K resistor. A number of the I/O pins overlap with the built-in push buttons as illustrated in the following diagrams:

If you connect an external key or switch to an I/O pin that overlaps with a built-in push button, the result becomes an “either or” key: the host computer that is connected to the response pad via the serial port cannot distinguish between the two.

This pin is not connected on the RB-420 and RB-620 models. The discussion below applies only to the RB-520 and RB-820 models.

Nearly all stimulus presentation software such as SuperLab “open” the serial port just before running an experiment, keep it open for the duration of the experiment, and then close it when done. The RB-x20 response pads have a built-in voltage regulator that keeps the voltage fairly steady at around 5.4 volts while an experiment is running.

But since the response pad does not have its own power supply and depends for its power on the computer’s serial port, what happens when an experiment is not running is entirely up to the computer and the way that it is designed. For example, on an HP Pavilion N5270 laptop, the measured voltage dropped to 5.16 volts. On a DELL Precision PWS420 workstation, the voltage even turned negative.

Conclusion: if you are going to rely on the voltage supplied by pin 7, it is a good idea to measure the voltage available both when an experiment is running and when it is not.

The RB-x20 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.

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-x20 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-x20 models, the timeout is 5 milliseconds. It is applied intelligently. For example, if 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.

Debouncing affects you in two ways. First, it is important to realize that we have deliberately left one input line on the Accessory Connector (AC2) without any debouncing on these two models: the RB-420 and the RB-620. This is convenient if, instead of a switch, you are connecting a signal generator, an I/O line from a computer, or any other input source that does not bounce.

Second, if you decide to connect a switch, it must be a good quality one because the debouncing time in the response pads is a short 5 milliseconds. Some switches have a bounce time that runs over 200 milliseconds. Also, on the RB-420 and RB-620, avoid connecting a switch to line AC2 since it is not debounced at all.