RS-232 Hardware Interface:
BOB-3’s serial data I/O signals can connect directly to the UART TXD/RXD pins of most microcontrollers, but you
need an inverting RS-232 hardware interface if you want to connect to a PC serial port or any other ‘true’ RS-232
host system. Here’s a PC COM port interface circuit example:
To BOB-3 pin 2
C2
0.1uF
U1
SP232ECP
15
16
13
8
10
11
1
3
4
5
2
6
12
9
14
7
GND
VCC
R1IN
R2IN
T2IN
T1IN
C1+
C1-
C2+
C2-
V+
V-
R1OUT
R2OUT
T1OUT
T2OUT
J1
DB9-FEMALE
1
2
3
4
5
6
7
8
9
C5
0.1uF
To BOB-3 pin 8
To BOB-3 pin 3
C1
0.1uF
C3
0.1uF
To BOB-3 pin 9
C4
0.1uF
In the RS-232 interface example above, the PC’s hardware handshake signals are looped back. This can simplify
application programming, but it’s not necessary if you know how to deal with each of those signals explicitly in
software. BOB-3 relies on software flow control, so the only host connections actually
required
are serial data in/out
and ground. J1 pin connections are for a standard male/female 9-pin serial modem cable with all pins wired straight
through. This hookup will not work with null-modem cables, or any other serial cables with internal cross-
connections.
The suggested Sipex SP232ECP (plastic DIP package) chip can use charge pump capacitors as low as 0.1uF.
Maxim’s old standard MAX232 requires 1.0uF, but the MAX202 works with 0.1uF. Chip pinout is identical. Even
though the Sipex chip incorporates 15KV ESD protection, you should add protective networks in the RS-232 signal
lines if your system environment isn’t benign. EMI filtering may also be required. Be sure to add a power supply
decoupling capacitor of at least 0.1uF across U1 pins 15~16 (C5 above).
Note that some industry-standard RS-232 interface chips do not support the highest communication bit rate now
offered by BOB-3. If you’re using one of the highest bit rates, be sure to check this specification as part of your
component selection process. Also confirm that cable capacitance does not exceed the chip’s drive capability at
your system’s maximum speed.
Many other circuit variations are possible, including the simple receive-only transistor circuit described in the BOB-II
literature, but most BOB-3 applications will benefit from bi-directional communication and software handshaking.
This is especially true at any of the higher baud rates.
Pin Description Table:
Pin Description
1
+12VDC Supply; 8~16V acceptable, 70mA required (plus pin 3 load)
2
Ground (power & digital); also wire to pin 29 or host board ground plane.
3
+5VDC
±5%
auxiliary output, or +5VDC regulated power supply input. 50mA maximum
load. Add decoupling capacitance if load current isn’t constant! +5VDC
±5% power
may
be applied to BOB-3 here if pin 1 is left open.
4
PD5; Reserved (do not connect)
5
PD4; Reserved (do not connect)
6
RX Enable; incoming data is ignored if this input is low. Useful when more than one BOB
is connected to a single controller. Weak internal pullup. (use is optional)
7
Vertical sync input from crawl circuit (use is optional)
BOB-3 Version 3.4 Application Guide ~ Copyright © 2004 by Decade Engineering ~ Page 4
