What kind of RS232 protocol uses just pin 2 and 4 on a DB-9 connector. This is what is sent from a Tenma model 72-6870 Digital Multimeter.
A quick Google search led me to a forum discussion on the topic of the 72-6870 protocol. Verbatim copy/paste follows. 

The Tenma periodically sends out a packet of 14 bytes of data without being
prompted. The parameters are 2400 bps, 8 bits, no parity, one stop bit. The
upper nybble of each byte contains the sequence number of the byte, starting
with 1. That is, the first byte (after a pause) is 0001XXXX, the next is
0010XXXX, the third is 0011XXXX, etc. The lower nybble is the data that
drives the LCD display. In the following, bit 7 is the most significant bit,
bit 0 is the least significant bit.

BYTE 1
BIT
7 - 0
6 - 0
5 - 0
4 - 1
3 - AC
2 - DC
1 - AUTO (RANGE)
0 - 1


BYTE 2 (LEFTMOST LCD DISPLAY DIGIT)
BIT
 7 - 0
 6 - 0
 5 - 1
 4 - 0
 3 - SIGN
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 3 (LEFTMOST LCD DISPLAY DIGIT)
BIT
 7 - 0
 6 - 0
 5 - 1
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT


BYTE 4 (SECOND LCD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 0
 4 - 0
 3 - DECIMAL POINT
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 5 (SECOND LCD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 0
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT


BYTE 6 (THIRD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 1
 4 - 0
 3 - DECIMAL POINT
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 7  (THIRD LCD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 1
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT

BYTE 8 (RIGHTMOST LCD DISPLAY DIGIT)
BIT
 7 - 1
 6 - 0
 5 - 0
 4 - 0
 3 - DECIMAL POINT
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 9  (RIGHTMOST LCD DISPLAY DIGIT)
BIT
 7 - 1
 6 - 0
 5 - 0
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT


BYTE 10
BIT
 7 - 1
 6 - 0
 5 - 1
 4 - 0
 3 - u (MICRO) INDICATOR
 2 - n (NANO) INDICATOR
 1 - k (KILO) INDICATOR
 0 - DIODE SYMBOL INDICATOR


BYTE 11
BIT
 7 - 1
 6 - 0
 5 - 1
 4 - 1
 3 - m (MILLI) INDICATOR
 2 - unused
 1 - M (MEGA) INDICATOR
 0 - TONE ANNUNCIATOR SYMBOL


BYTE 12
BIT
 7 - 1
 6 - 1
 5 - 0
 4 - 0
 3 - F (FARAD) INDICATOR
 2 - OMEGA INDICATOR
 1 - unused
 0 - unused


BYTE 13
BIT
 7 - 1
 6 - 1
 5 - 0
 4 - 1
 3 - A (AMPERE) INDICATOR
 2 - V (VOLT) INDICATOR
 1 - Hz INDICATOR
 0 - unused


BYTE 14
BIT
 7 - 1
 6 - 1
 5 - 1
 4 - 0
 3 - unused
 2 - DEGREES C INDICATOR
 1 - unused
 0 - 1