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Senior Member
If it is a 25-way D connector - and wired for RS232 - then only pins 2, 3 and 7 would be needed (ignoring the hardware handshake signals for now). Trace those 3 pins back and where do they go on P102?
If you measure the voltage between pins 2 and 7 and between pins 3 and 7 on the 25-way D with a multimeter set to DC volts (reading 25V or so full scale) what do you read? The RS232 transmit line should have a healthy voltage reading...
Dave
Senior Member
The simplest way to tell if it is wired for RS-232 (EIA) or TTY is to look at the number of links between the components P102, R145 and IC115. If there are 3 links - the card is configured for RS-232 / EIA mode. if only 2 links - it is configured for TTY.
You then need to connect the correct 3 wires of P102 for RS-232 / EIA operation.
P102 pin 4 (red wire?) = ground (pin 7 of the 25-way D).
P102 pin 5 (orange wire?) = data from the outside world to the H8 (receive data). This could be wired to pin 2 or 3 of the 25-way D.
P102 pin 8 (blue wire?) = data from the H8 to the outside world (transmit data). This could be wired to pin 2 or 3 of the 25-way D (but not the same as the above)!
Dave
Senior Member
Another thing to check: Do you need a Null Modem between the Heathkit and the terminal? It is a gadget that crosses the TX and RX pins (also key handshake lines) to interface correctly with terminal pins. It is needed if both ends consider themselves DTE rather than one a DTE and one a DCE. Very confusing.
3225.jpg
Senior Member
There are three, all connected to holes marked 'EIA'.Originally Posted by daver2
This is confirmed by DMM continuity test.
You then need to connect the correct 3 wires of P102 for RS-232 / EIA operation.
P102 pin 4 (red wire?) = ground (pin 7 of the 25-way D).
Also confirmed - to pin 2.
P102 pin 5 (orange wire?) = data from the outside world to the H8 (receive data). This could be wired to pin 2 or 3 of the 25-way D.
Also confirmed by DMM. Blue to pin 3 of the db25.
P102 pin 8 (blue wire?) = data from the H8 to the outside world (transmit data). This could be wired to pin 2 or 3 of the 25-way D (but not the same as the above)!
Working on voltage testing now.
Senior Member
On power up:
Pin 3 and 7 is 18V
Pin 2 and 7 is 0.05V.
When I try to transmit.. voltages do not change.
Last edited by falter; January 13th, 2018 at 12:25 PM.
Senior Member
So that makes sense.
Pin 3 of the 25-way-D is the TRANSMIT line (data from H8 to the outside world).
Pin 2 of the 25-way-D is the RECEIVE line (data to H8 from the outside world).
Just doing the washing up at the moment, so I will have a look at the next step shortly.
Dave
Senior Member
Just had a quick look.
Can you remove IC122? If you can, proceed as follows:
Connect pins 2 and 3 together on the 25-way-D connector (transmit to receive).
Measure the voltage (using your voltmeter) at IC121 pin 5 (red positive voltmeter lead) and IC121 pin 4 (black negative voltmeter lead).
Alternatively connect IC122 pin 8 to ground (pin 7) and then disconnect the link.
You should see the voltage on IC121 change from 5V to 0V as you insert and remove the link.
If not, move your voltmeter to measure the voltage on the 25-way-D connector (pins 2 or 3 with the red voltmeter lead and pin 7 with the black voltmeter lead).
Do you see the voltage changing now when you insert and remove the link on IC122 pin 8 to pin 7?
Remove the shorting link from IC122 and re-insert the IC (observing correct polarity).
Dave
Last edited by daver2; January 13th, 2018 at 01:04 PM.
Senior Member
Just thinking about it.. that's sort of backwards to what the standard is, isn't it? I thought RS232 standard was Pin 2 for TX and Pin 3 for RX. Or is this a convenience thing so you don't have to use a null modem?
Senior Member
It depends which piece of kit is the DTE and which is the DCE as to whether you need a null modem cable or not...
Who knows who wired it up originally and for what purpose?...
Dave
Senior Member
I've removed IC122 but not sure where the red lead is going on IC121?
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