Chapter 3 – Electrical Characteristics
Multi-Tech Systems, Inc. MMCModem GPRS Developer’s Guide (S000445D)
16
Serial Link Electrical Interface
A serial link interface is available complying with V24 protocol signaling but not with V28
(electrical interface) due to a 2.8 Volts interface. TX, RTS and DTR can be either 5V or 3V.
Data signals are Hi = Mark. Controls signals are Hi = Off.
The signals are Tx data (CT103/TX), Rx data (CT104/RX), Request To Send (CT105/RTS),
Clear To Send (CT106/CTS), Data Terminal Ready (CT108-2/DTR), Data Carrier Detect
(CT109/DCD), Ring Indicator (CT125/RI), and Data Set Ready (CT107/DSR).
Serial Link Pin Description
Signal
Pin #
I/O
I/O Type*
Description
CT103 / TX
25
I
CMOS
Transmit serial data
CT104 / RX
28
O
1X
Receive serial data
CT105 / RTS
30
I
CMOS
Ready to send
CT106 / CTS
13
O
1X
Clear to send
CT107 / DSR
27
O
1X
Data set ready
CT108-2 / DTR
29
I
CMOS
Data terminal ready
CT109 / DCD
5
O
CMOS / 2X
Data carrier detect
CT125 / RI
9
O
CMOS / 2X
Ring indicator
* See “Operating Conditions” in the section on the
50-pin Connector Description
.
Level Shifter Application for the Serial Link
Modem
Terminal
Level Shifter
Vcc
RI
DCD
RX
CTS
DSR
DTR
TX
RTS
Vcc
28
25
1
3
24
23
22
19
17
16
21
20
18
13
C1+
C1-
C2+
T1IN
T2IN
T3IN
T4IN
T5IN
R1OUTB
R2OUT
R3OUT
R4OUT
ON
C2-
T1OUT
T2OUT
T3OUT
T4OUT
T5OUT
R1IN
R2IN
R3IN
ERROR
OFF
27
2
26
4
5
6
7
10
12
8
9
11
15
14
S_RI
S_DCD
S_RX
S_CTS
S_DSR
S_DTR
S_TX
S_RTS
Vcc
Vcc
GND
Vcc
GND
MAX 3238
*This application note is valid for Vcc> 3.0 Volt (see MAX 3238 specifications)
Auto shut down mode is not used in this example.
Level Shifter Application Diagram for Serial Link
