This page is a collection of my notes notes about the DEC Rainbow, which I have recently started to play with.
Facts and trivia
Many applications refer to the keyboard function keys by label (eg “Resume”) rather than keytop legend (eg “F7”). The LK401 doesn’t have these labels, so it’s a good idea to use an LK201 instead.
The RX50 floppy drive uses the same flux-transition rate as conventional “double density” (ie 360KB) 5.25″ floppy drives. The only difference is that the RX50 has 80 tracks on one side of the disk (bottom side of disk) and no tracks on the other side. So the “tracks per inch” is doubled, but the storage capacity is the same as a conventional DSDD (360KB) floppy disk. So use DSDD media with the RX50. DO NOT use “high density” (ie 1.2MB) floppy disks with the RX50, as they are designed for a higher data transfer rate (500 kb/sec).
Connecting an RX50 Floppy Drive to a PC
The RX50 floppy drive can be connected to a PC. It is not straightforward though. Although the PC supported a “360KB 5.25” disk (and the RX50 happens to be a 360KB 5.25 disk) there are some important differences:
- The PC 360KB drive is a 40-track double-sided double-density 300RPM drive, whereas the RX50 is an 80-track single-sided double-density 300RPM drive
- The RX50 uses relatively conventional “Shugart” pinout on the 34-pin connector, whereas the PC significantly modifies the arrangement for pins 10-16
- The RX50 presents as two single-sided drives, whereas a conventional 5.25″ drive presents as a single double-sided drive
- The RX50 has a “TKG43” input (pin 2) whereas the PC does not provide a reduced-write-current output for higher track numbers
- The RX50 drive blocks transmission of the “TRACK 0 L” output, if the “SIDE” input pin is low
To physically connect the RX50 to a PC, you need a 34-way “straight through” cable, terminated by 34-way header plugs. If you use the cable “as is”, the RX50 lower drive will work as Drive B, and the RX50 upper drive won’t respond to the PC at all. To make both the upper and lower drives work, you need to modify the cable as follows:
- Mark one connector as “PC End” and the other connector as “RX50 End”
- Somewhere along the length of the cable, cut pins 10, 14 and 16 and bend the wires up
- Connect PC-End pin 14 (Drive Select A) to RX50-End pin 10 (DS0)
- Connect the anodes of 2 x 1N914 diodes to RX50-End pin 16 (/Motor On)
- Connect the cathode of one of the diodes to PC-End pin 10 (/Motor On A)
- Connect the cathode of the other diode to PC-End pin 16 (/Motor On B)
Note that this will give you a basically functional dual-drive RX50 (for example, for use with ImageDisk on a PC). But you will be left with the following limitations:
- The PC won’t natively recognise the RX50 (for example, as an MSDOS drive) because it doesn’t know anything about 80-track single-sided drives
- The RX50 ignores the “side select” input pin. So if you try to read from the RX50 as a double-sided drive, you will always get data from Side 0 no matter which side of the disk you are intending to read. Also, as mentioned above, the TRK 0 output pin will never go active (low). Similarly, if you try to write out a double-sided disk using an RX50 drive, data destined for either side will always end up on Side 0
- Because the RX50 is not smart enough to automatically reduce write current (on tracks greater than 43), you may end up with write errors on the higher-numbered tracks, as the PC won’t provide this signal
If you want to overcome the TG43 problem, consider using a PC-to-8-inch-and-rx50 adapter, such as the one from www.dbit.com. These adapters sit between the PC’s floppy disk controller and the physical RX50 (or 8-inch floppy drive) and drive the TG43 line appropriately.
Formatting a blank disk
If you are starting with a pre-formatted RX50 disk, you can just use the “FORMAT A:” command to make the disk usable in the Rainbow.
If you are starting with a truly blank disk (ie one that doesn’t have the RX50 formatting already laid down on it), you have 2 options:
- Use ImageDisk to create a blank RX50 disk (and then use the “FORMAT A:” command to make it usable in the Rainbow); or
- Use the Rainbow’s “FORMAT A:/I” command
Note: If you want to create a blank RX50 disk using ImageDisk, do the following:
- Go into “Settings”. Select one-sided disk and 1:1 interleave
- Select “Format Disk”. Then select Sectors/track = 10, Start sector = 1, Data rate/type = 250 kbps MFM, Sector size = 512
REGIS Graphics demonstration
One of my Rainbows came with some REGIS demonstration software already installed on the hard drive. That Rainbow boots into MSDOS 2.11 and the hard drive was partitioned into drives E and F. In E:\BATFILES there was a file called REGISDEM.BAT. In E:\REGIS there was a file called REGIS.EXE (which switches on the colour graphics functionality). In F:\REGISDEM there were 15 .PIC files and a file called DONE.SCR. Once in REGIS mode, you can send any one of the .PIC files to the display simply using the TYPE command.
I copied all the files listed above on to an RX50 floppy disk, then modified REGISDEM.BAT so that it would work with all the files being in a single directory. I did this by inserting the REM command (using the EDLIN editor) at the start of each line that wasn’t needed. I also REM’d out all the PAUSE commands, to get rid of the annoying “Strike a key when ready …” messages after each screen.
The resultant RX50 disk was imaged using ImageDisk and saved as REGISDEM.IMD.
Links
Diagram of floppy crossover cable This diagram shows how two conventional floppy drives are connected to a PC. This cable requires that both floppy drives are configured to respond as the second drive. Their position on the cable then determines whether they respond as Drive A or Drive B.
Transferring files to the Rainbow Post by Jeff Armstrong (PrintStar) about transferring files to the Rainbow via a serial cable.
Video connector pinout information Describes the various video cables used by the Rainbow (monochrome and colour) and explains how a dual-head cable could be made.
Will’s DEC Micro Personal Computer Page
Some IMD images of Rainbow disks” Includes REGISDEM.IMD referred to above.