Modem / Serial Port FAQ

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chuckphd53
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Modem / Serial Port FAQ

Post by chuckphd53 »

The JR has a slot for a 300 Baud modem and it has the Back Side Serial Port connector as part of it's design.

- The JR handles the COM port designation differently than other platforms, (big surprise)

If you have a system with a JR modem plugged in the modem is considered COM1 $3F8(address)
The back side Serial Port is then considered COM2 $2F8(address)

So using a comm program like Qmodem or others, all should be just fine.

But if you do not have an JR internal modem installed and want to use an external modem on the Serial Port, this is where you need to be aware.

When the internal modem is not installed, the JR now considers the back side Serial Port as COM1. No problem right! well......not exactly...

If you are writing a program in basic and open COM1 1200 baud to use the Jr Thermal printer, all is great you can write to the printer just fine..

Now hook up your Hayes Modem and start Qmodem to dial up your favorite 80's BBS, hmmmm something is not working, you set up Qmodem to
use Com1 but it is not working....

Here is the caveat to the JR Com/Serial port !

We know that Serial Ports are defined as:
COM1 - $3F8
COM2 - $2F8

But when the internal modem is not installed, and the back side serial port is used, it now becomes COM1.
But the difference is that the back side serial port hardware address still is fixed to the $2F8(address). See the issue?

To use the Back side Serial Port, you must adjust any parameters in say, Qmodem, to assign COM1 as $2F8 address to work properly.

Why then IBM Folks did this, I am not sure, maybe had to do with the Thermal Printer, and ease of use across different programs that wanted to use COM1 as the printer port. If you have the FAQ's on why please post and share.
“I never commit to memory anything that can easily be looked up in a book.” (AE)

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