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3000 DTCs and Serial port cables
Written by: Jerry Mills

The HP 3000/9000 violates the RS-232-C standard. The HP 3000 is a DTE type device (producer or consumer of data) but HP decided to wire it to look like a DCE device. This allows HP to connect terminals (DTE) to the computer using straight thru connections. Modems or DCE devices will require cross wired cables to work.

Rule of Thumb: DTE to DTE use a crossed cable (pins 2 and 3 crossed)

DCE to DCE use a crossed cable (pins 2 and 3 crossed)

DCE to DTE use a straight through cable

Direct Connect or Modem - HP refers to its serial port connections as either direct connect or modem.

Direct Connect requires only pins 2, 3 and 7 of the RS-232 connector. All HP printers, with the exception of 2601A and 2602A can be set up to only require pins 2, 3 and 7. Non-HP printers can also be connected with the direct connect 2, 3 and 7 pins. ATP ports come in a 3 pin variety that can only be direct connect but a port with a DBM25 pin can also be only direct connect. If a port has modem signals, configuring the software for direct connect will disable the modem controls.

Modem Connect typically is a DBM25 connector with 25 pins to contain the RS-232 control lines. The port must have the electrical circuits for the control lines or it is just a direct connect. For example the ATP 4x4 mux panel has 4 each direct connects, 4 modem connects and is marked accordingly. In addition the software must be configured to enable the modem signals.

Analyzing Typical HP DCE to DTE serial cables

40242M DCE (Modem) Interface to Terminal (the HP3000/9000 is the DCE device)

Length: 5.0 meters (3.24 x 5 = 16.2 Feet long)

Connectors are symetrical and are DB-25 with Male pins

Typical Usage: Connecting HP Terminals to ADCC, ATP, MDP Full Modem and DTC’s

Name on Connector: None

HP3000 Port Terminal SIGNAL

(DCE) signal name (DTE) SOURCE EIA

FG 1 ------protective ground------- 1 FG

TD 2 <----transmit data------------- 2 TD Terminal BA

RD 3 ------receive data------------> 3 RD HP3000 BB

RTS 4 <----request to send---------- 4 RTS Terminal CA

CTS 5 ------clear to send-----------> 5 CTS HP3000 CB

DSR 6 ------data set ready---------> 6 DSR HP3000 CC

GND 7 ------signal ground------------ 7 GND

DCD 8 ------data carrier detect-----> 8 DCD HP3000 CF

14 ------------------------------------ 14

15 ------------------------------------ 15

DTR 20 <----data terminal ready---- 20 DTR Terminal CD

23 ------------------------------------ 23

24 ------------------------------------ 24

{DB25-M} {DB25-M}

By observation you can see that the connections are simple straight throughs. Terminals can also be connected using 3 wires only.

Note: Control signals are active only when configured by the system; i.e. NMGR or as a modem sub-type. Keep in mind these signals can also be re-defined according to how the port was configured.


Analyzing typical modem cable (DCE to DCE)

40233A Modem Cable Wire List

HP3000 Port Modem

(DCE) signal ground (DCE) SOURCE

FG 1 ------protective ground------ 1 FG

RD 2 <----receive data------------- 3 RD Modem

TD 3 ------transmit data----------> 2 TD HP3000

RTS 4 <----data carrier detect----- 8 DCD Modem

CTS 22 <----clear to send------------ 5 CTS HP3000

DCD 8 ------request to send-------> 4 RTS Modem

DSR 6 ------data terminal ready--> 20 DTR HP3000

GND 7 ------signal ground----------- 7 GND

RI 9 <----ring indicator------------ 22 RI Modem

DTR 20 <----data set ready---------- 6 DSR Modem

{DB25-M} {DB25-M}

As can be observed the cable wiring is drastically altered when connecting a modem to a HP I/O port. Many of the wires are crossed to simulate otherwise missing or redefined signals caused by connecting a DCE to a DCE. We will analyze the cable by focusing on how to satisfy the modems inputs and outputs.

Modem Data

TD connect 3 to 2 data must be crossed over

RD connect 2 to 3

Modem Controls and Status

Hardware Handshake with RTS/CTS

RTS DCD from the HP3000 will simulate request to send.

CTS CTS from the modem will go to pin 22 of the mux port which has been redefined by the software

to be CTS input.

RI Ring indicator from modem goes to software defined pin 9 and alerts the HP3000 of incoming call.

DTR DSR from the mux port is used to simulate DTR and enable autoanswer.

DCD When modem completes its connection with the remote modem; RTS is used to

inform the HP3000 the modem is online with a carrier.

DSR is used to tell the HP3000 the modem is powered on and operational.

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