I rmemeber that somebody here was looking for info on the HP9877 'External Tape Memory'

This is a rackmount box with up to 4 tape cartridge drives on the front, which can be connected to a 9825 calculator (and maybe to other machines too). It can be used (I believe) to copy just about any kind of file between tapes, make distribution tapes, and so on.

Inside the box are 4 main assemblies : 1) The PSU (transformer + low voltage switching regulator for +5V + linear regulators for other voltages) 2) The tape drive(s) (1-4 units can be fitted)

3) The controller board(s). One board per tape drive installed. This handles motor speed control, and data conversion between a bitstream and the signal to the read/write electornics. And other misc functions like end-of-tape detection 4) An interface board that links to the controller boards and to the 50 pin Blue Ribbon connector on the back. Here's what I've discovered so far : Much of the insides came from the 9825. The tape drive and PSU are straight out of the 9825. The controller board nearly is -- there are a few very minor changes. In particular, the tape controller is the same to software as the one in the 9825. There are the same 8 'registers' (4 for input, 4 for output), with the bits in the same places. I would be suprised if 9825 and 9877 controller boards can't be interchanged, actually. That leaves the interface. Much of the elctronics on that is to provide the right sequence of signals to clear a couple of status flip-flops on the controller boards. It's a chain of monostables to provide a serquence of pulses that first selects all 4 controllers at the same time, and then accesses a couple of the 'registers' which have the single effect of clearing said flip-flops. The external connector is pretty much what you'd expect. A lot of the pins aren't used. Of the ones that are, you have : 8 bidirectional data lines, to talk to the controller registers (==IODn on the 9825 bus) 2 register select lines (== ICn on the 9825 bus) 2 drive select lines Assorted control lines (== IOSB, DOUT, DMAR, etc on the 9825 bus) Unit Enable (disables access to all controllers when low, so you could, in theory, have multiple 9877s on the same interface) Initialise (clears just about all controller registers) The interface is very low level. If you want to read a tape, you set the tape going and every so often you get a data request from the 9877. You then read one of the registers. Bit 0 is the data bit just read from the tape. THe 9877 does not pack the bits into bytes or words. I am told this unit links to a 98032 parallel interface. Well, it's possible, but it would be a bit of a kludge. That doesn't suprise me too much, as the whole thing is a bit of a kludge (it could have been made a lot more elegant if the controller board had been slightly redesigned).

For example, the chain-of-monostables on the interface board could have been replaced by a clear input to the flip-flops on the controller boards. There also appears to be no circuitry to directly copy between tapes without going via the 9825 host. This has been rumoured about, but my guess is that copying a tape uses a DMA buffer in the 9025. The 9825 will do a lot of things 'in the background', so it's quite possible that you could carry on using the machine for normal calculations while it was copying a tape, giving the impression that the 9825 wasn't really involved. If anyone has the details on how to link this to a 9825 (I guess it's the Option 077 for the 98032), then I would be interested in finding out more

-tony