Serial RS232 problem

[cjjeeper]

So I have a very old radio modem that uses hardware flow control. Its RS232 with RTS, CTS and RTD. I have pins:
2- Tx
3- Rx
5- GND
6- RTD
7- RTS

8- CTS


My new equipment, I have no control of imitating hardware flow control.



So I put a loop back on pins 7 and 8. Its still not "going" I don't know what to do with PIN6 (RTD) because there are no other pins to do a loop back with that. And by that I mean the DB9 does not have any other pins other than what I listed. (it's a custom made cable with the push through DB9 pins, if you recall those from back in the day).


Any ideas what can be done here to get this to go?


Thanks

 

[Ken Roach]

EDIT: Are you replacing the modem, or keeping the modem and replacing the equipment ?

What's the connected equipment/PLC ?

It makes sense that jumpering Ready To Send / Clear To Send (7 & 8) together resolves the usual buffer-limiting handshaking that a modem and a device do to make sure there's no overflow on the modem's buffer.

Sometimes Data Terminal Ready ("DTR", not "RTD") is a one-way signal, set by the transmitting equipment pin Pin 4 to let the modem know that there's data being transmitted.

But I'm a little confused on which port you're describing with this pinout.

I think that it's a 9-pin DTE port, because Pins 5, 7, and 8 are labeled correctly for a Data Terminal Equipment (DTE, usually the PLC or computer) side of a DB9 connector.

But for a DTE device, pin 2 is Rx and pin 3 is Tx, and pin 4 is DTR (set by the PLC, while pin 6 is DSR (set by the modem).

Do you have the ability to run the existing modem with a serial breakout/indicator box in line with the cable ?

The more I think about the problem I think that maybe I'm confused about the pinout and the old modem is setting Pin 6 (Data Set Ready) to indicate to the equipment that it's ready for a transmission.

So this equipment either needs to have DSR/DTR handshaking turned off, or you need to figure out what Pin 6 does during idle state and during transmit by the modem.

 

[cjjeeper]

Hey Ken,


Yeah the DCE is a RITON radio Modem. The DTE is a Modicon 984 through a MUX. I am replacing the MUX with a Digi Port PortServer serial modbus to TCP/IP Modbus on a compactlogix.



There is no pin 4 in the cable.


I don't have an analyzer but I have a breakout Terminal block for the DB9.



Yes I believe that the modem is setting PIN 6.



I definitely wanted to turn off the handshaking but so far have been unable to reconfigure the modem to do that. Its as if there is not a setting or combo of settings that allows that. The manual gives DIPSW settings to set delays to zero but that has not had an effect.


If the modem is setting PIN6, the PLC side doesn't even need that. But I think that because the RTS/CTS is not a toggle its not setting DSR in the modem.

 

[cjjeeper]

CORRECTTION


2- RX
3- TX
5- GND
6- DSR -data set ready
7- RTS

8- CTS

 

[Ken Roach]

Thanks for that detail and context !

For those of you with knees that don't hurt in the winter, the Modicon 984 was a very popular control platform about 30 years ago. A "Mux" is a BM85 communications gateway/mulitplexer device, that had four serial ports that could do various Modbus RTU serial things and bridge to Modicon PLCs on the proprietary Modbus Plus network.

Digi International was a leader in serial/Ethernet connectivity before they were in the IoT device business, and make several devices that have mutiple flavors of Modbus connectivity.

Exactly which Digi device are you using ?

Is the Digi + CompactLogix system going to be the Modbus RTU serial Master, or a Modbus RTU serial Slave ?

The PortServer TS1/TS2/TS4 do support Modbus RTU/TCP conversion (you have to dig a couple layers deep to find that specified) and also advertise "Full modem and hardware flow control, modem emulation, port buffering".

Maybe the modem really is waiting for at least a state transition on RTS/CTS and you'll have to enable that in the PortServer instead of jumpering it out.

If you're going to poke around with old serial ports, a jumper/breakout box is a great investment, and they're both small and cheap these days.

 

[cjjeeper]

I am using a digi Portserver TS MEI (1 port)
The compact logix is TCP/IP to the port server (using the AOI)
I have it configured for the correct connection. I can loop back from a Modbus TCP/IP master to a serial modbus slave on my PC. So I know my wiring is good and my configuration is good.



I set the digi port serial port to "hardware flow" control but no go.


I can run a serial interface test using Modscan32 with the hardware flow control set and it works with the modem. It needs the RTS as the first send. I cannot get the Portserver to send that. (obviously I have the MUX out of the picture)