You are correct about not going wrong with the 9600 cable, and no need for RS-232 variant. You can change the signaling levels of the DR Mobile on its circuit board if you need to, so not really an issue regardless:
Click here for details Understanding Rig Control Options – digirig
Band A and Band B Interactions and Data Modes:
To clarify for those who are not familiar with this radio; Yaesu also uses the term “Band” to refer to the two transceivers of this radio; Band A and Band B refer to these individual transceivers.
They are not referring to what I would normally expect the term to mean. To many, band tends to mean frequency band; e.g. VHF.
With this radio you can run Band A and Band B as VHF/VHF, VHF/UHF, UHF/VHF or UHF/UHF simultaneously, because of the two transceivers.
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Not that you’ve asked specifically, but for interest, while using the FTM-500’s built-in APRS MODEM on one Band, you can scan on the other, or use the other Band for non-packet at the same time;
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Yes: Using external packet with DR Mobile will work the same as the built-in APRS; i.e. you can be doing packet on one Band, while scanning or using the other Band for non-packet; and
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No: You do not have to turn anything “off” per se if you’re running packet data on one Band, and talking on the other.
Now to qualify item 3… If both Band A and Band B are on the same band (confused yet
) like VHF/VHF or UHF/UHF, the FTM-500 will blank the receive of Band A, only when/if Band B is transmitting, or vice versa. This is to protect the Band’s receiver from being overloaded by the transmit of the other Band, both being in the same band.
So in practice: If you’re in a long session connected to a packet node (just monitoring is not really affected) while both Bands are in the same band (e.g. VHF/VHF), the constant toggling of transmit/receive of the Band running packet will blank the receive of the other Band, making a QSO challenging, and the packet node connection miss received packets when you transmit in the QSO…
I run the built-in APRS (and external Winlink Express via DR Mobile) on VHF, Band B, and it’s quite usable when Band A is also on VHF. When I send Winlink, I’m interacting with the software, and the e-mail session is over quite quickly (more so with VARA FM**), so again, quite usable in practice.
However, the good news: If Band A and Band B are in different bands, such as VHF/UHF, then receiver blanking does not occur, and both bands can operate without interruption.
The operators who like satellite/Space Station modes like this feature, as they can monitor the downlink on one Band, on the downlink band, While their transmit occurs on the other Band, on the uplink band, as these bands are not the same; i.e. they are VHF/UHF.
The FTM-500 is fully configurable via the set-up menus as to how you want to handle packet data, on which Band(s).
Now, you, and everyone else who read the entire response, will have the word “band”, stuck in the brain for the next week 
Steve VE7IIF
**I have been very impressed with VARA’s software MODEMs, and if you think you’ll being doing any Winklink Express on VHF/UHF or HF, it’s been worth it to me to have paid the one time fee to register/unlock the full speed.
I seldom use conventional packet on any band because of how slow it is in comparison. A registered version of VARA allows Winlink use on any/all bands, and keyboard to keyboard comms on any/all bands. I’m not affiliated with VARA in any way - only a customer who is very impressed.