When is 300 baud not 300 baud and 1200 baud not 1200 baud? When we are talking about modems. Modems are a way to convert the serial communications into a bunch of tones to go over the phone line. These terminals use modems to talk to the various hosts to run transactions.

The Verifone terminals got started using 300 baud modems. Back then, that was an inexpensive way to transfer information. The amount of information being transfered was so small that 300 baud was very adequate. Later on, the terminals were upgraded to use 1200 baud which was just about as good.

In order for two modems to talk to each other, they have to follow some standard. Since the first terminals were sold in the USA, the 300 baud modems followed the "Bell 103" standard for communications. That means that the modems used specific tones to mean specific things (space, mark).

Often, an engineer could listen to the line and tell if something was going down the line. It was a simple way to do some quick testing of how the communications was going. The 1200 baud used different frequencies and followed the "Bell 212A" standard.

There are other standard besides the "Bell 103" and "Bell 212A". The International Telephone Union has different standards. "V.21" which runs at 300 baud, but uses different tones all the way through. "V.22" runs at 1200 baud and is likewise different. So, a 300 baud modem from Europe can not talk to a 300 baud modem from the US.

When two modems try to talk to each other, they start off by sending certain tones to each other.

Those of us that have heard our internet modems squeal know the noise of two modems trying to see what the other will listen to.

The problem we run into with 300 and 1200 baud is that a number of modems are not really comptible with each other. Many "Hayes Compatible" modems are not. One side will think that it has connected, but the other side does not. For example, we had one client who had to talk to a bunch of modems already installed in the field. The modems he had already purchased would not talk to those modems at 300 baud. The only way that they were able to get the communications going was that they went to the computer store and bought every model of modem they could find and test them. They found only one specific model that would talk to those modems in the field and then, they bought a bunch of that model. Later on, that model was not available any more.

We see this happen with the Verifone terminals also. I have a Practical Peripherals 2400 baud modem which works very, very nicely with Tranz and Zon Jr XL terminals. But, it does not work well with the Omni 3200 and 3750 terminals. All this is when trying to communicate at 300 baud. The 3750 will say that it has connected at 300 baud, but the communications will be garbage characters. I have a US Robotics 33.6 Fax Modem which works with the Omni 3200 and Omni 3750 terminals but not with a Zon Jr XL. I have a genuine Hayes 56K modem which works well with all of them. Similar problems occur at 1200 baud. We see this all the time nowadays. Many modems being shipped today are not tested well at these lower baud rates and do not work properly.

Just because a modem says that it supports 300 baud, that does not mean that it really does. Nor when a modem says that it is connected at 300 baud is it really connected. All modems need to be tested to find which ones work.

Dave Randolph,
President, Prairie Trail Software
1-800-618-4199

Back to 2004 newsletters

Back to newsletter main page