This is the VT78 to RX01/RX02 BC80D-5K cable pinout (from buzzing out mine)

Pins 1-9 of the DB25 go to pins 1-9 of the DB37
Pins 14-23 of the DB25 go to pins 20-28 of the DB37
Pin 25 of the DB25 is attached to the cable shield which is grounded
at the other end with a wire to one of the mounting screw on the DB37.

This is the pinout I got by buzzing out the adapter cable I made to go 
from the DB25 on my RX02 to the 40 pin cable from the RX8E. 
If the pinout was in a document I can't find it, I may of
gotten the pinout by buzzing out the cable in the drive.  
Pins 10,11,23, and 24 are not connected in the cable from my VT78 but  
pin 25 is present.  I don't remember why my adapter cable has
extra pins connected.

DB25    40 pin header
 1        C  (38)
14        D  (37)
 2        H  (34)
15        J  (33)
 3        M  (30)
16        N  (29)
 4        S  (26)
17        T  (25)
 5        W  (22)
18        X  (21)
 6        AA (18)
19        BB (17)
 7        EE (14)
20        FF (13)
 8        KK (10)
21        LL  (9)
 9        PP  (6)
22        RR  (5)
10        SS  (4)
23        TT  (3)
11        UU  (2)
24        VV  (1)

12,13,25 Not connected.

Looking into pins of 40 pin male connector numbering is

2 (A)     40 (UU)
1 (B)     39 (VV)

Signals in PDP-8 mode (most other pins are ground)

D : RX RUN L
J : RX ERROR L
N : RX INIT L
T : RX TRANSFER REQUEST L
X : RX DONE L
BB : RX DATA L
FF : RX 12 BIT L
RR : RX OUT L
LL : RX SHIFT L

Additional signals used for RX02 with PDP-11
TT : DRV AC L
VV : RX DMA