BAMHC4998 pg.
1/3
B&B Electronics -- Revised December 1998
RS-232, 422 or 485 Buffered Asynchronous
Modem Handshake Controller
Model BAMHC
Description
The BAMHC provides hardware handshake control for Radio Frequency
and other modems requiring the RTS line to control the transmitter. RS-232,
RS-422, or RS-485 data signals at up to 19.2K bps are supported. The BAMHC raises RTS and buffers up to 160
characters while waiting for the modem to be ready to transmit. The amount of time data is buffered is user-selectable for
5, 10, or 50 ms. Alternatively, the modem's CTS line can be used to indicate ready to transmit.
The BAMHC supports both the Transmit and Receive data lines, and the built-in microprocessor provides hardware
control of the RS-422/485 driver. The BAMHC enables the driver when it first sees data from the modem. It waits 10 bits
after transmission is complete, then disables the RS-422/485 driver. All connections to the host device are provided on
the same DB-25 female connector, while RS-232 modem connections are to a DB25 male. The unit is powered by 12
VDC and requires a maximum of 65 mA.
RS-232 Host Connections
Connection of the BAMHC is simple and straightforward. The DB-25 female serial connector is used for connecting to
either RS-232, RS-422 or RS-485. The RS-232 signals are pinned as a DCE device (input on Pin 2 and output on Pin 3.)
This means that a straight through cable can be used from your DB-25 port from any DTE device such as a PC or
terminal. A standard 9 to 25 pin adapter can be used in cases where the serial port on the DTE device is a DB-9. For
connecting to modems or other DCE devices, a null modem cable or adapter that swaps pins 2 & 3 is needed. See
Figure 1 for connection diagrams to 9 pin and 25 pin DTE and DCE devices. Note that because RS-422 and RS-485
signals are also available on the same connector, special care should be taken not to hook any external signals to these
pins. This should not be a problem for most serial devices, but if your device has power or special non-standard outputs,
a custom cable should be made that does not connect to the extra pins on the DB-25 connector. CTS is provided as an
output on pin 5 to tell the host device when buffer space is available in the BAMHC.
Figure 1: RS-232 Host Connection Diagrams
DB9 DTE Device
TD
RD
RTS
CTS
DTR
DSR
DCD
Signal GND
3
2
7
8
4
6
1
5
DB9 DCE Device
TD
RD
RTS
CTS
DTR
DSR
DCD
Signal GND
3
2
7
8
4
6
1
5
DB25 DTE Device
TD
RD
RTS
CTS
DTR
DSR
DCD
Signal GND
2
3
4
5
20
6
8
7
2
3
4
5
20
6
8
7
or
DB25 DCE Device
TD
RD
RTS
CTS
DTR
DSR
DCD
Signal GND
3 (RD)
2 (TD)
5 (CTS)
4 (RTS)
20
6
8
7
or
BAMHC DB25S
BAMHC DB25S
2
3
4
5
6
8
7
2
3
4
5
20
6
8
7
3 (RD)
2 (TD)
5 (CTS)
4 (RTS)
20
6
8
7
BAMHC DB25S
BAMHC DB25S
BAMHC4998 pg.
2/3
B&B Electronics -- Revised December 1998
Table 1: Dip-Switch Settings
Pos.
1
Pos.
2
Pos.
3
Pos.
4
Pos.
5
Pos.
6
Pos.
7
Pos.
8
1200 bps.
OFF OFF OFF
X
X
X
X
X
2400 bps.
ON
OFF OFF
X
X
X
X
X
4800 bps.
OFF
ON
OFF
X
X
X
X
X
9600 bps.
ON
ON
OFF
X
X
X
X
X
19.2K bps.
OFF OFF
ON
X
X
X
X
X
8 Data Bits
X
X
X
OFF
X
X
X
X
7 Data Bits
X
X
X
ON
X
X
X
X
No Parity
X
X
X
X
OFF
X
X
X
Even Parity
X
X
X
X
ON
OFF
X
X
Odd Parity
X
X
X
X
ON
ON
X
X
CTS to send
X
X
X
X
X
X
OFF OFF
5ms to send
X
X
X
X
X
X
ON
OFF
10ms to send
X
X
X
X
X
X
OFF
ON
50ms to send
X
X
X
X
X
X
ON
ON
X = Don't Care
RS-422 & RS-485 Host Connections
The RS-422/485 driver and receiver are connected to 4 pins on the DB-25 female connector. Signal ground is still on
Pin 7. When connecting to a four-wire RS-422/485 device or system, connect the output of your device to pins 16 (+) and
17 (-). The input to your device should be connected to pins 14 (+) and 15 (-). For two-wire RS-485 systems, the driver
and receiver of the BAMHC must be connected together by tying pins 14 and 16 together and 15 and 17 together. This
allows the BAMHC to communicate half-duplex over the same pair. Refer to Figure 2 for connection diagrams to your
RS-422 or RS-485 equipment.
If termination is needed, a space on the PC board of the BAMHC labeled R6 allows you to solder in a termination
resistor across the RD(A) and RD(B) lines. The off-state bias resistor values can also be changed by removing R5 and R8
and replacing them with through-hole components. Before making modifications to the BAMHC, be sure to consult the
B&B Electronics RS-422/485 Application Note or other sources of information to see if termination is necessary. The
Application Note is available from our Web site, or call and we will be happy to send one at no charge.
RS-232 Modem Connections
The BAMHC modem connector is pinned as a DTE device for direct connection to a modem with an RS-232 interface.
A straight through DB-25 connector or standard 9 to 25 adapter should provide all connections needed. The BAMHC
supports TD, RD, RTS, and CTS, with DTR tied to +5 V to provide this input to the modem if needed.
Setup and Operation
A dip-switch SW1 and jumper JP1 can be
accessed by carefully prying open the hood with a
small screwdriver. The dip-switch (SW1) defines the
data format and transmit wait time. Positions 1
through 3 on the switch determine the baud rate.
Position 4 sets the unit for 7 or 8 data bits. Position
5 enables/disables parity. Position 6 determines
Even or Odd parity, and Positions 7 and 8 set the
transmit wait time to 5 ms, 10 ms, 50 ms, or wait for
CTS. See Table 1 for Dip-Switch settings. The unit
must be powered down for dip-switch changes to
take effect.
The BAMHC waits for a character from the host
device. When it receives the first character, it turns
on the RTS line to the modem and either waits a pre-
determined time or until it sees CTS from the
modem. It buffers up to 160 characters during this
time, then begins to transmit to the modem while still
receiving data from the host device. When all data
has been transmitted, the BAMHC waits two
character times before lowering RTS.
JP1 sets the BAMHC to either full or half duplex when used with RS-422/485 host devices. With the jumper in place,
the RS-422/485 receiver is disabled during transmission, preventing data from being echoed back to the modem in RS-
485 two-wire configurations. Removing JP1 leaves the RS-422/485 receiver enabled at all times, allowing full duplex
communications in four-wire systems.
Figure 2: RS-422/485 Host Connection Diagrams
422/485 4W Device
TD A (-)
TD B (+)
16 RD B
17 RD A
14 TD B
15 TD A
RD A (-)
RD B (+)
7
GND
BAMHC DB25S
485 2 Wire Device
Data A (-)
16 RD B
17 RD A
14 TD B
15 TD A
Data B (+)
7
GND
BAMHC DB25S
BAMHC4998 pg.
3/3
B&B Electronics -- Revised December 1998
Power Connections
The BAMHC runs off 10 to 16 VDC power. The unit itself draws approximately 15mA. This does not include external
loading of receivers or termination. Each RS-232 driver may be loaded up to 4mA and termination on the RS-422/485
driver could bring the total to up to 65 mA. Power can be connected in one of three ways. The 2.5mm power jack
provides an easy connection to an external supply such as B&B's model 232PS. The tip is positive. Power can also be
brought into the unit on pins 25(+) and 12(-) of either DB25 connector. A protection diode inside the device protects
against reverse polarity connections, but care should still be taken when connecting power.
Specifications
Transmission Mode:
Asynchronous, half or full duplex.
Host Interfaces:
RS-232, RS-422, or RS-485
Modem Interface:
RS-232
Data Rates:
1200, 2400, 4800, 9600 or 19.2K bps
Data Formats:
7 or 8 data bits, Even, Odd, or No parity (7-N-1 Not Supported)
Time to Transmit:
User-selectable for 5 ms, 10 ms, 50 ms or wait for CTS.
Host Handshaking:
RTS/CTS or None.
Connectors:
DB-25 Female for host connection, DB-25 Male for modem connection
Power Supply:
Requires 10 to 16 VDC @ 65 mA max.
Dimensions:
4.1"L x 2.3"W x 0.95"H
Figure 3. Circuit Block Diagram
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