At The Terminal : Problems with communications

By Paul Zabrs
The Australian Apple Review Vol 3 No 4 PP11

Paul Zabrs

EVEN THOUGH communicating with microcomputers is becoming very common, it may still be quite daunting for the novice if he strikes problems.

Few things are more disappointing than when, after spending several hundred dollars on peripherals, you turn everything on and nothing happens. The tendency is to try several times and, if the system still does not work, to give up in frustration and anger for having spent all the money.

Provided you have good equipment, there is no reason it should not work. Of course, there is always the possibility of faulty equipment, but in my experience this is rare. The failure to communicate properly has usually a simple cause.

Maybe the cable?

The most common problem is in the cable connecting the computer with the modem. The usual manifestation of such a problem is that you are unable to communicate with anything at all. Fortunately, it is practically impossible to cause any damage to either the computer or anything else by using an improperly configured cable. Things simply won’t work until the cable is right.

What do you do then? With most modems only three of the 25 pins of the usual DB25 connector need be connected, namely pins 2, 3 and 7. Pin 7 on the computer side is invariably connected to pin 7 on the modem side. Because of lack of standardisation, depending on the interface card and modem, pins 2 and 3 are sometimes connected to their equivalent pin numbers on the other side, at other times they are crossed, i.e. pin 2 is connected to pin 3 and pin 3 to pin 2.

To test the theory of the wrong cable, simply cross pins 2 and 3 on one side and try again. Another point is that with the CC7710 card pins 4, 6 and 20 should be shorted on the card side.

Data format Another common problem involves the selection of data format. Most remote computers are happy with 8 data bits plus one stop bit. However, some require a specific data format, e.g. 7 data bits, 1 parity (even or odd) bit plus one stop bit, otherwise they will not acknowledge what you are typing. To rectify the problem it is simply a matter of setting the software to the required data format.

Half and full duplex The problem of half and full duplex is common and usually trivial to solve. When full duplex communications is required and you are set for half duplex, all you type appears on the screen twice. No harm done, just switch the software to full duplex. If you are in full duplex and the host requires half duplex, all your typing is invisible. To rectify the situation, simply switch to half duplex.

Remote services When connecting to remote services such as mainframes, bulletin boards, Viatel etc, your modem should always be switched to ORIGINATE mode. If, on the other hand, you are trying to establish a link with a friend with a similar system to yours, it is essential that one of you be in originate mode and the other in answer mode. That’s what the modem switches are for. It does not matter which side is which, provided there is one of each in a communications link. Almost all remote services are always in answer mode.

I have concentrated on problems with easily rectifiable causes. There is one problem with the older Apple llc’s (purchased prior to December 1984) which manifests itself in intermittent garbage on the screen and is not amenable to any immediate solution. It is caused by a design flaw (long since rectified) and those people who experience this problem should see an authorised Apple dealer for a free fix. Communicating with your Apple should be as reliable as with any other equipment. Proper software and hardware should obtain good results for both hobby and professional use.

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