IBM PCjr. Modem - What can I do with it?

Hardware questions and modifications

Re: IBM PCjr. Modem - What can I do with it?

Postby jmetal88 » Tue Feb 18, 2014 3:47 pm

Brutman wrote:Do you have local echoing turned on in Procomm?

Yeah, it sounds like you have your keys echoing locally, then being echoed by the BBS. So what you see instantly is generated on the PCjr, then what you see after a bit is the echo from the BBS. I bet if you turned off local echoing, the problem would disappear.
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Re: IBM PCjr. Modem - What can I do with it?

Postby GHiero » Tue Feb 18, 2014 4:03 pm

Brutman wrote:Do you have local echoing turned on in Procomm?

Changing the duplex setting to full duplex seems to have solved the double character problem. This can be done with Alt + E. The solution was in the Procomm manual, but was 115 pages and I wasn't sure what to search for.

Procomm is much more reliable and can display color ANSI characters compared with the built in BASIC terminal emulator. Still, I cannot wait to get back to faster modems and I maintain that the PCjr.'s Internal Modem is most useful as a COM redirector.

I hope my adventures with the IBM PCjr. Internal Modem will be instructive to somebody. What I have to say also applies to the IBM PC Convertible Internal Modem, which is an updated version of the PCjr.'s Internal Modem (see my blog for more similarities) and any other Novation 103 Smartmodem based device.
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Re: IBM PCjr. Modem - What can I do with it?

Postby Brutman » Tue Feb 18, 2014 4:48 pm

"Full duplex" or "half duplex" refers to the ability of the modems to send and receive data at the same time. I don't think I've ever run into a half duplex modem; every modem I've ever seen has supported full duplex operation.

The attribute you are referring to is local echoing vs. host echoing. Local echoing is useful when talking to a device or system that does not echo each character back to you. Most BBS systems and things that you would connect to provide that service so it is often not needed.

I recommended Procomm because I know it works well, and it works with the PCjr internal modem. It was just a matter of getting the commands from the technical reference manual. There is a trick you missed that is particular to the Novation command set; certain patterns of binary data in files can drop the modem back into command mode. This frustrated the hell out of people when they tried to download files that had the magic pattern in it. There is a command in the Novation command set to tell it to ignore the magic sequence of bytes so that your downloads do not get hit by it.

Your adventures might be useful to somebody, but to anybody who experienced these things the first time around we are just chuckling. You've actually been led down a fairly well beaten path; it's just been a while for most of us. Hence my ability to point you to so many parts of the puzzle ...

Faster modems make it more tolerable but really Ethernet is the ultimate solution. The stock BIOS on a PCjr can't program the UART for more than 4800 bps and the clock divisor that the PCjr used is slightly off compared to the one on the PC so that any software that tries to program the UART directly is using the wrong values, unless patched for a PCjr. Combined with the keyboard interrupt handling and the lack of DMA, the poor machine can only run faster than 2400 bps if you don't use the keyboard, diskette drive, or make screen updates. Which is fine for downloading a file to a ram disk, but that's about it. Ethernet adapters provide vastly more buffering which alleviates those problems.
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Re: IBM PCjr. Modem - What can I do with it?

Postby geoffdaddy » Tue Feb 18, 2014 11:00 pm

Back in the day, full/half duplex were common vernacular and were used interchangeably with local echo on/off, but Mike is right: the technical term means that the modem can send/receive data at the same time in full duplex and not in half. I have seen a couple of terminal programs refer to full/half instead of local echo. In practice, the only time I recall using local echo was when using GEnie.
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