You also need a pitch-to-MIDI converter, usually referred to as a guitar/MIDI interface.
Roland has a few, and as far as I know is the only company currently producing decent quality MIDI interfaces. Some of theirs have built-in sounds (GR-55, etc.), some are strictly pitch-to MIDI converter that require an additional sound source (GI-10, GI-20).
Axon had the most highly regarded guitar/MIDI interfaces, but they are out of business.
The Roland GK 13 pin layout is the current standard for all units - Roland, Axon, Yamaha all use(d) this kind of cable between the guitar and the interface. Many years ago, each manufacturer had their own - Roland used to have a 24 pin, etc.
These things come up used on Ebay a lot, so I'd look there rather than buy new. Pitch-to-MIDI converters have not really improved much in the last 5-10 years.
All the 13 pin style converters are much better than the Sonnus stuff, but they're still far from perfect.
The best way to get bass guitar MIDI is with a hardwired controller like a Peavey CyberBass/DataBass (discontinued, pretty rare) or the stuff by industrialradio.com (expensive).
You can get decent live performance use out of any of these, but from what I've heard using them to get MIDI data into a sequencer is usually not so good because a lot of the bends and slides get translated into large amounts of pitch bend information by the interface, which makes quite a mess in a sequencer.
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