68KMB Tech Notes

68KMB Tech Notes

Revised: February 15, 1996

The following is a collection of technical notes for the 68KMB. They are provided for students or teachers using the 68KMB package (including the 68KMB computer, the I/O boards, and the associated software). If you find any errors or omissions, please contact Scott MacKenzie (mac@snowhite.cis.uoguelph.ca).

Table of Contents

Tech Note #1: Interfacing to a 9-pin COM port connector

If your system has a 9-pin COM port connector, then the serial cable provided with the 68KMB will not work. You can build a new cable, or add an adapter to the existing one.

If you build your own cable, here's what you need:

  (PC-end)       (68KMB-end)
    DB9S            DB25P
     2 ------------- 3
     3 ------------- 2
     5 ------------- 7
If want to use an adapter with the cable supplied, just use a standard 9-pin-to-25-pin adapter for a serial mouse. If you bought your mouse separate from your system, you probably got an adapter with it. Just put this on the PC-end of the cable and you're in business. (see also, Tech Note #2)

Tech Note #2: Mouse driver interference

If you are interfacing to a 9-pin COM port connector which is also used as a port for a serial mouse, you may have a few problems. Some users tell me they do not have a problem. For others, it appears that PC-VT works fine, except when you try to load a hex file from the disk, or when you escape to the DOS shell. You may get garbage on the screen. The cure is to disable the mouse driver in the AUTOEXEC.BAT file. Just insert "REM" before any line in AUTOEXEC.BAT that references the mouse, and re-boot your system. Things should work fine thereafter.

Tech Note #3: Speeding-up the 68KMB

If you don't like the relatively low operating frequency of 3.6864 MHz, you can speed-up the 68KMB with a few simple changes. Here's what you have to do:

1. Remove the 3.6864 MHz crystal labeled XTL1. It's near pin 33 of the 68000 IC. Try to remove it intact, because you'll need it in step 3.

2. Insert and solder a new crystal (e.g., 8 MHz) in place of the one just removed.

3. Insert and solder the 3.6864 MHz crystal in position XTL2, near pin 1 of the 68681 IC.

4. Insert and solder two ceramic capacitors, of about 15 pF each, in positions C25 and C26, near crystal XTL2.

5. Remove jumpers XA and XB, located near the large jumper block by the power connector.

1. If you wish to go above 8 MHz, this is possible but you will need a new CPU chip as well. I've got a system working fine at 16 MHz (without changing any other components). JDR Microdevices (1-800-538-5000) sells 10, 12, and 16 MHz 68000 CPUs.

2. After making the changes above, everything will work pretty much as before except your programs will execute faster. The 68681 is still clocked at 3.6864 MHz, so the serial interface and timer are not affected. The only programs affected are those using software delay loops (e.g., ROTATE, LED5HZ, and COUNT4). The delays will be shorter, so things are a bit faster than before.

3. If you are burning your own EPROMs to run on a 16 MHz 68KMB, use 150 ns devices. Verify that the RAMs are 150 ns or faster, as well. If you get intermittent "hang" conditions with garbled output on the display, either the RAMs or the EPROMs are too slow.

Tech Note #4: Reducing the size of listing files

If you would like to keep your listing files short to minimize the amount of paper wasted, here are three tricks for you:

1. Put the assembler directive

in the first line of the source program. This will supress the use of page breaks in the listing file.

2. Edit the file A.BAT and remove the "X" switch near the end of the command line. This will supress generation of a symbol table, thus further shortening the output listing.

3. Put the assembler directive

just before ASCII strings or other data constants defined using the DC directive. Subsequent output to the listing file will only contain one line for each source line. Extra bytes are still placed in the object file, but they will not appear in the listing file. This directive can be used anywhere in the source file; but I find it most useful for the definition of lengthy ASCII strings. LSTCOD+ will restore things to normal.

Tech Note #5: Using a regulated power supply with the 68KMB

If your lab environment has regulated power supplies and you wish to operate the 68KMBs using them, no problem. Have a look in the User's Manual you received with the 68KMB. The 68KMB schematic is at the end. The power connections to the 68KMB are shown at the bottom of the last page.

In case you don't have the manual handy, here's what you need to do. Locate the white power connector near the plug for the AC adapter. It's labeled J5. This is the connector to which a regulated power supply can be connected.

To bring power in on J5, you'll have to build a cable with the appropriate mating connector. You can get one from Radio Shack for about $2.50. It's an ARCHER "6-conductor male molded nylon connector" (PN 274-226).

Only two of the six connections are used. Here's what you need:

 Regulated              J5
Power Supply         Connector
   +5V ---------------- pin 4
  GROUND -------------- pin 2
You must also move the jumper on the 68KMB near J5 from "INT" to "EXT". And that's about it.

Thanks to Ashok Kumar Srinivasan of Tennessee Technological University for suggesting this technical note.

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