Campbell SDM-SIO4 User Manual - 1.2

Section 1 .  Introduction

1-3

1.2

 Specifications

1.2.1

Serial Ports

The SDM-SIO4 has four serial ports which can be configured independently

to use different serial data formats and baud rates (from 25 to 115,200

baud). These ports are 0-5V logic or ±5V for RS232 and are configured

similar to a PC ‘AT’ style DTE serial port.

Handshaking, to control the flow of data to and from a sensor, can be done

by the datalogger or SDM-SIO4 if needed, and can be in the form of

hardware or software protocols.

Table 1  SDM-SIO4 Serial Port Pin Configuration

Pin No.

SDM-SIO4 Port

1.

RI ring indicate/DCD in

2.

RX in

3.

TX out

4.

DTR data terminal ready out

5.

Ground

6.

DSR in

7.

RTS request to send out

8.

CTS clear to send in

9.          

 +5V if internal link fitted, otherwise no connection

If you have an older SDM-SIO4 which has  female ‘D’ type

connectors, your connections will be different from those

shown above. Please either refer to your earlier Manual or

contact Campbell Scientific for further details.

Serial Port Buffers

Each serial port has a receive (Rx) buffer, a transmit (Tx) buffer and a

processed data storage buffer. It is important to understand these buffers as

their size can determine how often data must be collected from the

SDM-SIO4 by the datalogger. It is important to avoid letting these buffers

fill up. They are of the ‘fill and stop’ type, i.e. if they fill up, and more data

is sent into the buffer, the extra data will be lost.

The receive and transmit buffers for each port are 981 bytes long and there

is an additional 16-byte hardware buffer for each port.

The processed data storage buffer (used to store converted data ready for the

datalogger to collect) is 891 bytes long, which is large enough for 222 4-

byte Campbell Scientific floating point values (refer to the datalogger

manual for more details of this format).

There is one more buffer, which is used only when the datalogger outputs

floating point data via the SDM-SIO4. This buffer is 241 bytes — long

enough for 60 floating point values. (The size of this buffer is rarely a

limitation as it is emptied quickly.)

NOTE