Monday, November 13, 2017

HP minicartridge tape drive successes


2645A terminal and tape drives

I printed the instructions for working with the two minicartridge tape drives built into my terminal, went out to the lab and attempted to follow them. I was quickly aware that the keyboard on my terminal is labeled differently than the manual. This is most apparent in the function and device control keys. 
The key labels on my terminal
The key labels that should be there
The good news is that the physical position is what matters, not the labeling. That is, if I push the button marked READ on the keyboard shown in the manual, it does indeed command the tape drive to read its file contents and display it on the screen. Thus I can use my terminal even with the mislabeled keyboard.

Based on that, I did some testing with tapes inserted into the left and right drives. I wrote some text on the screen, then did a RECORD of that onto the left tape drive. Issuing REWIND brought it back to load point and then a READ did recreate the screen contents. 

READ from the right tape, containing nothing, produced no output on the display. I then did a REWIND for both drives followed by a COPY FILE from left to right. REWIND then READ of the right drive now produced the contents I had recorded on the screen, just as it did with the left drive I originally recorded on. The drives and tapes work just fine.

I did rearrange keycaps to put the eight function keys in their correct location. Now, with a bit of careful labeling, the keyboard will be fully usable.

2622A terminal testing

I pulled the cable off the interface card and opened the hood to check that the wiring was correct, matching the documentation for the cable that should be used with the terminal. Everything was perfect and all wires had connectivity between the HP 1000 end and the Centronics connector end.

I tried some variations in data communications configurations on the terminal, but nothing seemed to get characters to display when I started the tape diagnostic asking it to use the terminal for conversational mode.

The program was looping outputting characters to the 12966A interface card, but nothing completed. My next set of tests will make use of the oscilloscope and/or logic analyzer to watch the lines between interface card and terminal.

First likely problem is a configuration mismatch, but it is possible I have bad components in the terminal. I will check that hypothesis by temporarily connecting to my 2622D terminal, but this will wait until tomorrow. I have other obligations today.  


Marc Verdiell has a large collection of different HP minicartridge drives scattered across a number of HP terminals, computers and devices. The transports for these drives are varied, both mechanically and in how the tapes are used. For example, some devices use two channels while the 2645A uses only a single wider recorded channel on the tape. 

The HP85 has a different capstan size, uses dual channels and has room to be modified to support the somewhat more reliable and modern DC2000 minitape cartridges. That modification involves increasing the height of the capstan, or in some cases milling a groove and using an O-ring to move the tape. 

There are several ways to deal with all these drives:

  • continue using original DC100 cartridges
  • switch to Athana remanufactured DC100 cartridges ($$$)
  • switch to using DC2000 cartridges

 One can restore the capstan and make use of DC100 cartridges, as long as you can find media whose drive bands have not degraded. This is hard to accomplish. One can modify the drives to use the wider tape of the DC2000 cartridges, but the coercivity of the media is different and needs a change to write current to work well. One can use remanufactured DC100 tapes from Athana, but these also need a write current increase to work.

The DC2000 route seems obvious except when you find that some of the drive mechanisms don't allow the bigger cartridge to fit without some draconian changes to the enclosures they come in. This is true with the 2645A terminal, whose bezel would need ugly hacking. HP 85, 9825 and some other units easily allow the DC2000 to be used. There are some HP instruments that use the tapes as well and some of them don't have the room for the DC2000 clearances.

Most DC100 tapes available, even new-old-stock sealed in plastic, have their drive bands so degraded that the snap and even worse, stick to and remove oxide where they have sat in contact over the years. 3M brand tapes have the best record, but only relatively. I have a batch of them that seem to be working find in my 2645A but even these break in the 9825 whose transport is more aggressive moving tape. 

Marc discovered a way to recondition DC100 tapes to work well even with the rough treatment of the 9825 transports. First, he opens the cartridge and discards the existing drive band. Next, he reams out the rollers for the drive band slightly and cleans then oils the pins they turn on. Finally, he uses 4 1/4" Plastibands as the replacement drive belt.

With all those changes, the DC100 cartridge will work reliably and with its original media coercivity, doesn't need a change to the write bias on the transport. This allows him to continue using DC100 in the 2645A and other drives where DC2000 modification is infeasible.

This also provides a way to rescue existing tapes with data written on them, as long as the degraded original drive band didn't pull off any tape oxide. Hopefully he can gain access to prerecorded material and programs that sit on DC100 cartridges. 

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