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[Lisa2]

At the VCF Midwest show last week I was displaying a running Lisa card cage containing a full set of Sapient recreation boards.   While the I/O board was a recreation, it had an original MW+II PFG board installed, and many folks pointed it out and had questions about the PFG.   There seemed to be some confusion about the PFG and what it’s for.   

The PFG is a small board that attaches to the Lisa I/O board that is part of the MacWorks Plus II package.  Here is a quote from the MW+II manual:

MacWorks Plus II includes a user-installable hardware upgrade called the X/Lisa Programmable Frequency Generator (PFG). The PFG circuit board has been designed to provide three major hardware improvements to the X/Lisa. These enable MacWorks Plus II to read virtually all valid 800k Macintosh disks, store Control Panel settings more effectively, and allow the serial ports to operate more closely to the Macintosh Plus hardware specifications.

I was asked if having the PFG installed required me to run MW+II on my Lisa.  The answer is no, the PFG has no effect if MW+II is not running.  LOS, Xenix, UniPlus, MacWorks, and MW+ all run fine with a PFG installed. 

I was also asked if running MW+II required more RAM, a SCSI disk drive, or the Lisa Screen Mod kit.  The answer is no, MW+II will work on any Lisa2 or Lisa2/10 with the standard 1 Meg of RAM.   Using 800K floppy disks will require a double side drive.

Running MW+II requires a PFG to be installed.  If you would like to try MW+II more information is here: https://lisalist2.com/index.php/board,11.0.html

If you would like to buy a PFG, you can buy one here:  http://vintagemicros.com/catalog/lisa-pfgprogrammable-frequency-generator-p-325.html

If you have any questions about MW+II or the PFG, please PM me and I will help anyway I can.

Rick

[AlexTheCat123]

The answer is no, the PFG has no effect if MW+II is not running.

There's actually a really slight effect when MW+II isn't running, but it's so minor that it's not really anything to worry about.

The PFG serves three main functions within the Lisa:

1. Modification of the floppy disk controller clock. Normally, the floppy controller is clocked at 2MHz, but the PFG injects its own clock into U6A using two clip leads in order to override this default. By varying the floppy controller clock to a variety of values slightly above and below 2MHz (under software control from MW+II itself), the PFG allows the Lisa to read many Mac disks that weren't readable under the stock 2MHz clock. You can actually use the PFG with the clips disconnected and it'll still work just fine, but you'll get a warning about it on startup and of course you won't get the floppy-reading improvements. Since the clock only gets changed at the request of MW+II, this upgrade is completely inert when running another OS.

2. An additional 256 bytes of PRAM. The Lisa has very little PRAM by default (it just uses some left-over space in the floppy controller's shared memory), and the info stored in PRAM will be lost if your I/O board batteries die or your system is unplugged if you don't have batteries. The PFG stores everything in an EEPROM (the little 8-pin chip on the PFG board), so now it's non-volatile, and it provides a lot more space to allow you to store a lot more Control Panel settings. This is also how MW+II remembers your startup configurations (like whether or not you have an XLerator installed and what mode you want to start the XLerator in). As with the floppy controller clock modification, this upgrade is completely inert when running other operating systems.

3. Modification of the SCC clock from 4MHz to 3.672MHz. This one is pretty simple! The stock Lisa clocks the SCC at 4MHz, but the PFG changes this clock to 3.672MHz (the same frequency as the Mac Plus SCC) for better compatibility with the Mac serial ports. Unlike the other two upgrades, this one is actually active all the time, even when you're running a different OS. This has never seemed to cause any problems, at least from all my testing, but I think it's definitely worth pointing out that the PFG does at least slightly change things when running other operating systems!

It's great to see that PFGs are available for sale again!

[sigma7]

There's actually a really slight effect when MW+II isn't running, but it's so minor that it's not really anything to worry about.
...
3. Modification of the SCC clock from 4MHz to 3.672MHz. This one is pretty simple! The stock Lisa clocks the SCC at 4MHz, but the PFG changes this clock to 3.672MHz (the same frequency as the Mac Plus SCC) for better compatibility with the Mac serial ports. Unlike the other two upgrades, this one is actually active all the time, even when you're running a different OS. This has never seemed to cause any problems, at least from all my testing, but I think it's definitely worth pointing out that the PFG does at least slightly change things when running other operating systems!

More details:

The Lisa's SCC has two sources of a baud rate clock, the "P Clock" and a crystal.

The crystal is of a frequency that facilitates baud rates with 0% error but it is only available as a baud rate source to the B port.

The P Clock is 4 MHz in the stock Lisa, which results in errors of about 0.1% when programming baud rates up to 19200 -- See "Figure 2-10 Serial Port Baud Rates" in the Lisa Hardware Manual.

The Mac uses a 3.672 MHz P Clock which gives baud rates with 0% error. The Mac does not have a separate crystal, so P Clock is used to generate the baud rate for both serial ports.

When designing the PFG, the choice was made to set the PFG's P Clock to 3.672 MHz so that the Mac software that directly accessed the SCC hardware would work properly.

So with the PFG installed, Serial B will work correctly with Lisa software that uses the crystal as the baud rate source, and both ports will work correctly with Mac software that uses P Clock as the baud rate source. However, Lisa software that uses the P Clock as the baud rate source for Serial A will not be generating the correct baud rates.

Due to the baud rate error that results from using the 4 MHz P Clock in the stock Lisa, it is preferable to use Serial B (especially at higher speeds), but in the event that you must use Serial A with early software (eg. Lisa Terminal), then you will need to remove the PFG (or adjust the baud rate at the other end to an unusual number to compensate).

edit: corrected magnitude of Serial A baud rate error and added reference to Hardware Manual