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Interfacing the SBC to a Personal Computer

The bidirectional serial interface which appears at the bottom of the microcomputer schematic is designed to connect the microcomputer to a personal computer. In this kind of connection we have to make the distinction between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE).

Generally the equipment which controls the communication is considered DTE. The PC is in that category. A MODEM or a printer with a serial interface is considered DCE. It is the intermediary or terminus for the DTE. The designation has a bearing on the configuration of the serial connector used with each device.

DTE equipment may use either a 25-pin or 9-pin D-subminiature (D-sub) male connector. The DTE equipment utilizes pin 2 of the 25-pin male connector for sending data, pin 3 for receiving data and pin 7 for ground. It uses pin 3 of the 9-pin connector for sending data, pin 2 for receiving data and pin 5 for ground. Most PCs have one serial port of each type.

The microcomputer is DCE equipment. A 3-wire flexible cable, about 4 feet in length, should be used to connect the RS-232 serial signals coming from the MAX233 chip to either a 25-pin or 9-pin D-sub female connector. The above means what it says. You don’t mount the connector on the board but on the end of the flexible cable. The 25-pin female connector should have pin 2 connected to the RS-232 input on the MAX233 chip, and pin 3 connected to the RS-232 output from that same chip. Pin 7 is connected to ground. The 9-pin female connector should have pin 3 connected to the RS-232 input on the MAX233 chip, and pin 2 connected to the RS-232 output from that same chip. Pin 5 is connected to ground.

On the latest PC compatible computers, both serial ports utilize use the 9-pin configuration. So it is preferable to use a 9-pin D-sub female connector for the end of the SBC cable. If you have already installed a 25-pin connector on the SBC then you can always use a 25-pin to 9-pin serial port adapter.

The baud rate can be set at something reasonable, such as 9600 baud, and the PC communication software should be set to match that rate. If all is wired correctly then communication should be established immediately.

The most straightforward program to use for communicating with the SBC is PROCOMM. The earlier public-domain versions are the best. To use it properly you must set the baud rate, using Alt-P, to correspond to the baud rate of the SBC. You should choose 8-bits, no parity and 1-stop bit. You’ll probably be using serial communications port 2.

To be able to download files into the SBC you must set the ASCII communications options using Alt-S. Go down to the ASCII TRANSFER OPTIONS and select 100 ms for character pacing, 1/10 sec for line pacing and make the pace character 0. (New designs of the SBC operate very well with zero delays.) For the other options choose NO or NONE.

Under TERMINAL OPTIONS go to GENERAL OPTIONS and select

A-  Terminal emulation ................ ANSI

B-  Duplex ............................ FULL

C-  Software flow control (XON/XOFF) .. OFF

D-  Hardware flow control (RTS/CTS) ... OFF

E-  Line wrap ......................... OFF

F-  Screen scroll ..................... ON

G-  CR translation .................... CR

H-  BS translation .................... NON-DESTRUCTIVE

I-  Break length (milliseconds) ....... 350

J-  Enquiry (ENQ) ..................... OFF

Now you will be ready to communicate without problems.

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