Comfusing between RS232 capble: straight and null-mordem caplble!

hi everybody,
I just bought a frequency counter (FC) machine of BK Precision-1856D. I'm trying to communicate with it. I used a RS232-USB capble to connect my latop to FC, when I sent a command D1 (data request command) to FC, it displayed that receiced a command, but I didn't get nothing from FC. Then I read its manual again, and I realized that it needs a straight RS232 capble, an DTR voltage must be 12V from PC, RTS voltage must be -12V from PC. I'm wondering about the different between RS232 straight and null-mordem capble. When do we use a straight calble, and when to use a null-mordem capble?. Why do we need DTR and RTS signals? My laptop doesn't have a COM gate, so what must I do to communicate with FC?

hi,
this is null modem cable connection.

This is RS232 communication with BK Precision 1856D guide document View attachment RS232_VB6_18XXX.pdf. Can anyone have any ideas for this? thanks,

Straight-through RS232 cables are used to connect DTE (Data terminal equipment, like a PC) to DCE (data communications equipment, like a modem or other device like your FC). So a straight through connection is what you require.

Null-modem (or Cross-Over) cables are used to connect two similar (DTE to DTE, or DCE to DCE) types of equipment together; typically two PCs. The difference is that the receive and transmit wires have to be swapped around, as do the control wires like DTR, RTS etc.

Anyway, you need a straight through connection. That is what your USB-Serial converter gives you.

Your problem is likely that the USB-Serial convertor does not give the full RS232 voltage levels. Most such convertors do not. Instead, they rely on most equipment just using logic levels, which is often all they need.

Your FC though says it needs the full +/-12V signals, probably because it uses them to power the serial communications (it is possible to extract a little power from DTR/RTS if the proper levels are used). That is quite unusual I think for a commercial device, but the pdf does specify the 12V levels so it looks like it is the case.

Can you try an older PC with a real serial port? And a straight through cable.

Failing that, talk to BK and tell them that they need to join the modern world and use a USB interface!

In some cases you can jumper certain pins, to fool the device so it will send, and other pins so it will receive.

This is a situation where you need help from BK, or from other people who have done this sort of thing.

These serial interfaces are a pain to get right, but if you're lucky it's a one time job and then it will work for you after that.

FoxyRick said:

The difference is that the receive and transmit wires have to be swapped around, as do the control wires like DTR, RTS etc.

Click to expand...

Hello FoxyRick,
Can you please explain the above line with more details ?

Thanks for your time.

Failing that, talk to BK and tell them that they need to join the modern world and use a USB interface!

Click to expand...

If they don't manage, they should at least try to keep the RS232 specification and not set arbitrary voltage specifications.

It's not clear what's the actual interface voltage requirement. You should also measure the RTS and DTR voltage against ground in operation. It may be the case, that the DTR and RTS states are not setup correctly. Receive will probably work without the auxilary voltages.

A picture is worth a thousand words:



The RS232 connections are arranged in a PC and a modem connectors so that the RX of one goes to the TX of the other, using a straight-through cable. Likewise for RTS/CTS etc. The connections are crossed over by the wiring in the equipment (DCE or DTE), not in the cable.

If connecting two PCs together a cross-over (null-modem) cable is required to do the swap over instead because, obviously, the sockets on the PC's are wired identically.

thanks all, I'll by a straight capble an try again. I'll back soon and show you the result.