|Interesting find: HP 9875A dual cassette drive|
Message #1 Posted by David Ramsey on 2 Nov 2011, 5:56 p.m.
I took a chance and bought this HP 9875 Cartridge Tape Unit from a recycled equipment company recently.
It's an HPIB device designed to be used with the 9820, 9825, 9830, etc. with the appropriate interface, so the back panel will look familiar to those of us who've used HPIB devices before.
The 9875A is an "intelligent" peripheral with its own processor and memory. This explains the giant circuit board inside. In fact according to HP it could be connected directly to HPIB measuring equipment and accumulate data on its own without a host computer. Flipping a "Self test" switch on the back will cause the unit to perform write and read tests on cassettes in both drives and blink a drive light if any errors occur.
Inside are two 9825A-era cassette drives that use DC100 tapes. Fortunately I have one good DC100 tape! Both tape wheels were goopy, but the good 'ol "heat shrink tubing" fix, inelegant though it may be, fixed that.
Being expensive, 70s-era HP equipment, it's built like a tank, with a thick aluminum frame supporting all components.
A clever feature: angled mirrors and clear ports above the tape drives allow you to see the label on the tape, if any, or the spinning tape reels if not.
Although the device predates the HP-85, it's a lot easier to play with on an 85 (with the optional HPIB interface) or HP-87 than on earlier machines, because these let you do BASIC things from the command line, like dimension string variables to hold status results, which can only be accomplished inside a program on earlier machines.
Although "intelligent", the 9875A doesn't have anything in the way of file system smarts, nor is it supported by "mass storage" ROMs. When you insert a tape, the first thing you need to do is send a "rewind" command, because otherwise the unit doesn't know where the tape is. You create files, specifying the number of records and bytes per record, and can write to and read from files or records programmatically. Various operations let you select optional read-after-write verification, select drive 0 or drive 1, and retrieve error codes, which are typically delivered as a single long string consisting of comma-separated numbers you must then parse yourself.
You can't (easily) save or load programs from the unit; and using data files is quite tedious: for example, you must programmatically position the tape to the start of a file with the "find file" command before reading or writing to the file; and files have no names and there's no concept of a "directory", so you'd better make notes.
Still, it's an interesting little bit of HP history, and the only one I've ever seen. $150 well spent.
Edited: 2 Nov 2011, 5:58 p.m.