Nuts & Volts - September 2006

BY MICHAEL SIMPSON

Adaptive website at www.adaptivedisplays.com.

The signs use a protocol called Alpha Sign Communications Protocol. You can find a complete description of the protocol at www.amsi.com/Pages/ 97088061.htm

The protocol can be a bit complicated as it supports networking devices. What I’m going to do in this article is exploit a feature of the protocol to create a very simple interface.

Programming Protocol

I have been doing PC and Pocket PC development for some time now and, a while back, I decided to create a simple development platform that would make interfacing to devices and robotics much quicker and easier. With the help of other engineers, we came up with a development platform called Zeus. Zeus has both desktop and Pocket PC versions available.

I have created a version of Zeus just for Nuts & Volts readers. ZeusNV will let you debug, compile, and test the code in this article. You can even create a stand-alone executable that you can distribute to others.

ZeusNV has one requirement. You must have the .Net Framework installed on your Windows machine.

The code we used is quite simple and can be adapted to just about any programming language that supports RS232 communications.

The Alpha Protocol

The Alpha protocol has some pretty advanced features such as networking and message labeling. It also supports uploading graphic images call DOTS. In order to make the explanation and examples as simple as possible, we will drastically simplify the interface by using certain simple aspects of the protocol.

Before we send a single byte to the sign, we need to open up a comport with the correct settings. The protocol supports the following baud rates: 1200, 2400, 4800, and 9600 with the following settings: 7 Bits, 1 Start, 2 Stop, and Even Parity. We use the Zeus ComOpen command to open the com port.

Const Ch1 1

ComOpen(Ch1,baud=9600,port=1,parity=2,bits= 7,stop=2)

Zeus can control five Async connections at once. These are called channels; hence, the Ch1 constant. You will need to change the port=1 setting if you are using anything else. For instance, if you are using Bluetooth on an IPAQ Pocket PC to connect to a Bluetooth RS232 connector, you will use port= 8.

Once the port is open, we have to tell the sign we are

■ FIGURE 2

ready to communicate with it. I call this the ready header.

ComOutput Ch1,chr(0)+chr(0)+chr(0)+chr(0)+chr(0)

ComOutput Ch1,chr(1)

ComOutput Ch1,”Z00”

We start with five null characters (Value 0). This sets the sign’s baud rate to the rate you are transmitting. (Note that if you are dealing with a language that can’t send Null characters, you may also send five 1s.) We then send an SOH (Start Of Header, Value = 1) character. After this, we can do many things but, in our case, we are going to send “Z00.” This tells the sign that we are addressing all signs on the bus.

The Alpha protocol supports sending checksums. However, to make the interface simpler, we are not going to utilize this feature.

With the header sent, we can now send a command. The Alpha protocol supports several command types, but we will only be using a couple of them. In this case, we are going to send a Priority

Text Message. What _■_{FIGURE 3}is neat about this message is that it bypasses the internal file system so it’s simple and fast. The down side is that you can only send 125 bytes, but since we are going to be sending real time messages, this is perfect.