PC Compatibility Card: Limited RS-232 Protocol Support

TECHINFO-0020680

TOPIC

I have a MS-DOS program that uses serial communications. It works properly on my desktop Pentium PC Compatible, however it does not work on the Apple PC Compatibility Card installed in my Power Macintosh 7200 computer. The software developer states the problem is related to the way the Macintosh supports the RS-232 protocol. Is there a difference between RS-232 protocol in the Pentium PC and the PC Compatibility Card in my Macintosh?

DISCUSSION

Macintosh computers implement the RS-422 protocol, so not all RS-232 signals are available. These RS-232 signals are not available: Carrier Detect (CD), Data Set Ready (DSR), Request to Send (RTS), and Ring Indicator (RI). If your application or serial device requires these signals, it will not work.

Neither the PC Compatibility Card or the DOS Compatibility Card support Carrier Detect (CD). You must configure your communications applications so they do not use CD. Most communications applications can be set to respond to the CARRIER string sent back by most modems.

A workaround for RTS and DSR (required for hardware handshaking flow control) is for you to configure your software to use XON/XOFF for flow control. If your software does not allow for XON/XOFF, then the software will not run on the PC Compatibility Card, or the DOS Compatibility Card.

In Windows 3.x, flow control is set in individual applications. Refer to application documentation on how to configure these programs.

The Windows 95 Modem control panel has an option to set flow control which most Windows applications will follow. To change the flow control follow these steps:

Step 1

Switch to the PC environment

Step 2

Open the Modem control panel.

Step 3

Select your modem, click on the General tab, and then click on the Properties button.

Step 4

Click on the Connection tab and then click on the Advanced button.

Step 5

Click Flow Control and make sure Software (XON/XOFF) is selected. Make sure Hardware (RTS/CTS) is deselected.

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