© Microhard Systems Inc.
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Appendix A: Serial Interface
DCD
Data Carrier Detect
- Output from Module - When asserted (TTL low), DCD informs the DTE that a
communications link has been established with another MHX 920A.
RX
Receive Data
- Output from Module - Signals transferred from the MHX 920A are received by the DTE
via RX.
TX
Transmit Data
- Input to Module - Signals are transmitted from the DTE via TX to the MHX 920A.
DTR
Data Terminal Ready
- Input to Module - Asserted (TTL low) by the DTE to inform the module that it is
alive and ready for communications.
SG
Signal Ground
- Provides a ground reference for all signals transmitted by both DTE and DCE.
DSR
Data Set Ready
- Output from Module - Asserted (TTL low) by the DCE to inform the DTE that it is alive
and ready for communications. DSR is the module’s equivalent of the DTR signal.
RTS
Request to Send
- Input to Module -
A “handshaking” signal which is asserted by the DTE (TTL low)
when it is ready. When hardware handshaking is used, the RTS signal indicates to the DCE that the host
can receive data.
CTS
Clear to Send
- Output from Module -
A “handshaking” signal which is asserted by the DCE (TTL low)
when it has enabled communications and transmission from the DTE can commence. When hardware
handshaking is used, the CTS signal indicates to the host that the DCE can receive data.
Notes: It is typical to refer to RX and TX from the perspective of the DTE. This should be kept in mind when
looking at signals relative to the module (DCE); the module transmits data on the RX line, and receives
on TX.
“DCE” and “module” are often synonymous since a module is typically a DCE device.
“DTE” is, in most applications, a device such as a host PC.
Module
(DCE)
Signal
Host (e.g. PC)
(DTE)
1
DCD
IN
2
RX
IN
3
TX
OUT
4
DTR
OUT
5
SG
6
DSR
IN
7
RTS
OUT
8
CTS
IN
Arrows denote the direction that signals are asserted (e.g., DCD
originates at the DCE, informing the DTE that a carrier is present).
The interface conforms to standard RS-232 signals, so direct connection
to a host PC (for example) is accommodated.
The signals in the asynchronous serial interface are described below:
